A review on the research status and development trend of equipment in water treatment processes of recirculating aquaculture systems
Ruichao Xiao1, Yaoguang Wei1,2 , Dong An1, Daoliang Li1,2, Xuxiang Ta1, Yinghao Wu1and Qin Ren1
1 College of Information and Electrical Engineering, China Agricultural University, Beijing, China
2 Beijing Agricultural Science and Technology Network Engineering Technology Research Center, Beijing, China
Correspondence
Prof. Dr. Yaoguang Wei, China Agricultural University, P.O. Box 63, Tsinghua East Road 17#, Haidian District, Beijing, 100083, China.
Email: [email protected]
Received 17 January 2018; accepted 7 June 2018.
Abstract
Recirculating aquaculture systems (RASs) are intensive aquaculture facilities models that depend on diverse water treatment equipment to maintain good water quality and produce safe and healthy high-quality aquatic products. This article combines the main farming-mode of water purification recirculating pro- cesses with recent cultivation-mode scientific research and the current develop- ment of the recirculating aquaculture industry. Harmful substances are present in aquaculture wastewater due to large quantities of residual particulate matter such as residual feed, faeces and small suspended solid particles, as well as ammonia, nitrite, bacteria and carbon dioxide (CO2), in the water. These harmful substances seriously affect the quality of aquatic products, so water treatment equipment is needed to remove these substances, add oxygen (O2) to the water and adjust the temperature of the water to ensure a high-quality environment for fish survival.
This article reviews the equipment for physical filtration (e.g. solid–liquid separa- tion equipment, microscreen drum filter and foam fractionator) that could remove suspended solids during the water treatment of RASs and the equipment for biological filtration (e.g. fluidized sand biofilter (FSB), moving-bed biofilm reactor (MBBR) and rotating biological contactor (RBC)) that could remove ammonia nitrogen, nitrite and other hazardous substances from wastewater, as well as equipment for water disinfection and sterilization, O2 addition, CO2
removal and temperature control. Comprehensive analysis and discussion of water treatment efficiency are provided for reference to create efficient high-end recirculation aquaculture models and increase the precision and intelligence degree of recirculating water treatment technologies in the future.
Key words: biological filtration, equipment, physical filtration, recirculating aquaculture system, water purification, water treatment technology.
Introduction
The top three problems that humans currently facing are population, resources and environment; with limited natu- ral resources, the international community must meet the urgent need for food and nutrition for an increasing popu- lation (FAO, 2012). As an important source of proteins and essential nutrients, the aquaculture industry plays an increasingly important role in the global food problem.
According to the report from China’s National Bureau of Statistics in 2016, there was 49.379 million tons of aquatic
products from aquaculture production, which accounted for 73.7% of the total annual aquatic products in China;
moreover, the average annual growth rate in recent years has been approximately 4% (Ministry of Agriculture, Fish- eries and Fisheries Administration, 2016). Over the past 50 years, China, representing the development of the aqua- culture industry, has developed faster than the global popu- lation growth (FAO, 2014). Aquaculture uses minimal grain in exchange for animal protein and thus has been acknowledged by international experts as the most efficient technology for acquiring animal protein (Liu 2011).
However, traditional aquaculture has many problems, including (i) poor infrastructure and weak economic foun- dation, (ii) water quality environment worsen, (iii) serious secondary pollution, (iv) severe damage to aquatic resources, causing ecological imbalance (Liu and Liu 2012).
Recirculating aquaculture systems (RASs) are made pos- sible through modern biology, environmental science, elec- tromechanical engineering and information science, and use mechanical and biological filters to remove faeces, residual feed and other solid particles, as well as ammonia nitrogen, nitrite and other harmful substances from aqua- culture water, then sterilize water, add oxygen (O2), remove carbon dioxide (CO2) and adjust the temperature, then water recycle to culture tanks to reach an average daily water recirculation more than 95%, and aquaculture water recycle many times and can significantly improve aquacul- ture efficiency (Wang 2003), RAS according to the main characteristics of aquaculture workshops and water purifi- cation equipment, which has the advantages of being less influenced by climate change, conserving water, high-den- sity intensive, contributing less pollution to the environ- ment and having easy control of emission. Recirculating aquaculture has experienced rapid development in recent years, it is prevalent among fisheries worldwide, RASs according with the requirements of sustainable develop- ment and are regarded as an inevitable trend of aquaculture improvement for the future. To this end, the Food and Agriculture Organization of the United Nations puts for- ward to promote efficient intensive aquaculture, repre- sented by RASs in both core and frontier areas (Sun & Liu 2016).
Recirculating aquaculture systems originated in the late 1960s, the typical ones were Japanese biological purification kit static water aquaculture systems (using gravel as med- ium) and European packaged multistage water aquaculture systems (Wuet al.2008). In the 1970s, due to research and development of rotating biological contactors (RBCs) and increased pretreatment before biological purification to fil- ter out particle contamination, the load on biological filters was reduced (Lin 2011). In the 1980s, appeared ‘unified’
mode of industrial farming in Europe, which was charac- terized by lower investment, easy management, better eco- nomic and environmental benefits and formed the main fish farming industrialization mode. Starting in the 1990s, the technology of biological engineering, microbial, mem- brane and automation control was gradually applied in RASs for water purification, bottom discharge, increasing O2 content and temperature control. Modern RASs have adopted almost all available water treatment processes and technologies (Lin 2011). As a modern mode of intensive aquaculture, RASs already have the advantages of high-den- sity culturing without being limited by season, water or land availability and of being environmentally controlled.
RASs are highly regarded by many developed countries;
they have also shown their support from the aspects of pol- icy, legislation and finance to promote RAS development (Sun & Liu 2016). At present, the countries most heavily using recirculation aquaculture are carrying out research on ecological engineering, water recirculation equipment and the efficiencies of recirculation water farming, as well as their corresponding equipment, mechanization and information technologies and facilities are the focused research goal.
Recirculating aquaculture in China was first evolved from flow water aquaculture, in the 1960s, land-based fac- tory aquaculture began in the industrialized fry rear, from
‘utility sheds+underground seawater’ to gradually expand to seawater fry rearing and aquaculture, the system level has gradually increased (Lei 1998; Liu et al. 2015a). RAS research began in the 1980s, used international advanced technology for reference, independent researched and developed a batch of seawater recirculating aquaculture equipments to suit Chinese national conditions, such as microscreen drum filters, ozone (O3) generator and protein skimmer (Liu 2011; Liu and Liu 2012; Zhu et al. 2009, 2012, 2014). In recent years, China has further perfected new RAS technologies, materials for water purification technology and equipments, multifunction solid–liquid separation devices, multifunctional protein separators, ultraviolet (UV) sterilizers, dissolved oxygen (DO) equip- ments, and water quality purification processes and tech- nologies were gradually improved (Shi 2009; Sun & Liu 2016; Wang 2017).
The key to the sustainable development of recirculation aquaculture is to improve the water recycling efficiency and reduce the level of wastewater discharge. In RASs, culture wastewater contains many particles such as residual feed and faeces and suspended particulates, which account for 80–90% of the total particulates by weight, and it also con- tains ammonia nitrogen, nitrite and a large number of bac- teria (Chen et al. 1993). These particulate matters and harmful substances seriously affect the survival of fish, so water treatment equipments are needed to remove these substances to ensure that fish have a good living environ- ment. Current RAS equipments are various, and difficult to operate, and must be linked to physical filter to remove suspended solids, to biological filter to remove harmful substances such as ammonia nitrogen and nitrite and equipments to sterilize, add O2, remove CO2 and adjust water temperature. Each piece of equipments has its own characteristics in form and assembly. Through RAS resource utilization improvement, the aquaculture industry is moving towards standardization, mechanization, intelli- gentialize and intensification of the ecological efficiency of culturing. This article reviews a large number of studies about water treatment technologies of RASs and provides a
reference for the further development of water treatment equipments in RASs.
Physical filtration equipments of RASs
In RASs, wastewater contains large amounts of residual feed, excrement and other solid particulate matters that need to be removed as much as possible in the early stage of water treatment (generally uses physical filtration to filter out impurities) to reduce the organic load in subse- quent water treatment. Physical filtration refers to filter- ing without chemicals to purifying water. The solid particles in RASs include inorganic (sand or coarse sand) and organic (faeces and biological flocculation) matter.
Organic matter in general contains volatile suspended solids (VSSs), which are particularly important for aqua- tic culturing, and involved in O2 consumption during organic compounds degradation and biofouling problems (Ni & Zhang 2007). The total suspended solids (TSSs) may remain in the farming system for a long time, which could have adverse effects on the fish, such as direct damage to the gills, blocking the biological filters, ammo- niation to produce ammonia nitrogen and consumption DO by particulate matters decay in the water. According to sedimentation performance (mainly determined by density and particle size), suspended matter could be fur- ther classified. The diameter of nonprecipitable TSSs is generally 1–100lm, which need to be removed with equipments; and that of precipitable TSS>100lm, which can be precipitated but are precipitated more efficiently by solid–liquid separation equipments (Ni & Zhang 2007;
Dolan et al. 2013; Yuet al. 2014; Fernandes et al. 2017).
The proportion of particulate matter in RASs was 1.05 (Timmons et al. 2002). Chen et al. (1993) found the specific gravity of solid particles to be 1.19; these small particles accounted for 40–70% of TSSs by weight, and the average density of small particles was close to the water density, which was easy to disperse. The question of how to timely remove TSSs in RASs become key for water treatment, as this removal directly determines the water quality and the stability of the system.
As environmental regulations become stricter, environ- mentally sound waste management and disposal practices are becoming more important in all types of aquaculture.
Suspended solids are the most important pollutants of aquaculture water and will indirectly increase biochemical oxygen demand (BOD) and O2 cost and influence fish health. Therefore, the rapid removal of solid wastes is the basic principle of RASs. In addition, it could use a filter or drum filter to remove larger particles, and slow sand filtra- tion and foam separation to remove smaller particles. Many different kinds of solid removal equipments have been developed in recent years, such as swirl separators,
parabolic screen filters (PSFs), microscreen drum filters, and foam fractionators.
Solid–liquid separation equipments
The simplest method for removing residual feed, fish exc- reta and other large particles from aquaculture wastewater is using sedimentation pond, but its efficiency is lower than solid–liquid separator. As the first water treatment unit of a RAS, the solid–liquid separator uses centrifugal force and gravity to remove large particles to avoid blocking or break- ing down subsequent processing units and can also reduce local head loss and conserving energy. The subsequent equipments include swirl separators and hydrocyclones. In aquaculture, these units typically remove particles 80 lm in diameter and larger, which represent 80% of the total particulate loading (Ebelinget al.2006; Ni & Zhang 2007;
Pfeifferet al.2008; Sharreret al.2010).
Swirl separators are easy to install and simple to manage, and have a better efficiency in removing large-particle sus- pended solids. Davidson and Summerfelt (2005) found that the radial-flow settler provided an approximately twice greater TSS removal efficiency than did the swirl separator at an identical size and surface-loading rate. Pfeifferet al.
(2008) found that the swirl separator could remove more than 90% of grains>250 lm in diameter in a tilapia (Ore- ochromis niloticus) RAS. Pfeifferet al.(2008) found that the removal efficiency of the swirl separator was over 90% for particles larger than 250lm in a two-tank RAS (28 m3) utilized for tilapia warm water production. Timmonset al.
(2001) stated that when compared with a traditional set- tling basin, a properly designed swirl separator could greatly reduce the footprint area needed to settle solids.
Veerapen et al. (2005) validated swirl separators with diameters of 0.6 and 1.5 m by performing experiments and by carrying out computational fluid dynamics (CFD) simu- lations. They found that compared to tank height and inlet position, separation performance had a major influence on outlet geometry, inlet diameter and tank diameter.
A hydrocyclone uses centrifugal force and a free vortex to remove solid particulate matter from aquaculture wastewater. When sewage at tremendous speed spirals downward flow, forming centrifugal force, the suspended solids are separated and injected into the lower mixed flow in the lower cone tip of hydrocyclone and then discharged.
Hydrocyclone has advantages of low initial investment, no maintenance, small size, ease to use, no energy consump- tion and low operation cost. Scott and Allard (1984) indi- cated that hydrocyclone prefilters removed over 87% of the suspended particulate matter>77lm in diameter in RASs.
New filters have been studied to improve separation effi- ciency and reduce energy consumption. Gu et al. (2010) designed a swirl solid filter by combining a particle filter
with the cyclone principle and a medium filter. Simulation tests showed that when TSSs were present in water at 50–
70 mg L1, the average particle (precipitated particulate matter >100 lm and some of the suspended particulate matter<100 lm in diameter) removal rate reached 87.2%.
Shiet al.(2017) investigated the relationship between sepa- ration efficiency and hydraulic retention time (HRT) of a hydrodynamic vortex separator (HDVS) applied in recircu- lating biofloc technology system using the CFD method.
The results showed that under three different HRTs, the floc removal efficiency of HDVS was lower than the simula- tion results, and with the decline in HRT, the experiment and simulation results both declined. Lee and Jo (2005a,b) reported low-pressure hydrocyclone (LPH) separation per- formances for two different sizes of polystyrene particles, which were similar to faeces from young common carp (Cyprinus carpio) in different densities and settling veloci- ties. When using a commercial LPH, various feed particles exhibiting different physical properties could be used to test the separation performance. Lee (2010) found that the sep- aration performance of LPH for suspended solids removal was selective, with the maximum particle size removed being 300–500lm in diameter in a RAS. Then, Lee (2014) tested the separation performance of an LPH with a lower inlet pressure (average 1.38–5.56 kPa) and using fine organic particles from 1 to 700lm in diameter, and used the response surface method model to predict an optimum operating inflow rate of 30% and underflow ratio of 721 mL s1for the LPH at maximum total separation effi- ciency. In 2015, Lee (2015) found the separation efficiencies of LPH were determined to be 60–63% for the faecal solids and 59–71% for the feed wastes in common carp and Nile tilapia (Oreochromis niloticus) RAS.
Some researchers have designed different solid–liquid separation equipments that better remove suspended solid particles. Liu (2001) designed a cloth bag filter to overcome the shortcomings of frequent backwash and waste water for conventional pressure filters. Although its suspended parti- cle removal efficiency is good, its precipitation of large par- ticles is not ideal; and the bag needs to be replaced frequently (Sharreret al.2010). Different flocculants have obvious effects on the filter processing of wastewater. Shar- reret al.(2009) used three kinds of flocculants (alum, ferric chloride and hydrated lime) combined with a Hychem CE1950 polymer to form 3 sets of geotextile filter bags, and found that all three sharply increased the capture of sus- pended particulate matter and that alum was the most cost-effective. Although the bag filter has a good suspended particle removal efficiency, in order to efficiently capture and dehydrate suspended particulate matter to determine the water flow, feeding quantity and suspended particle drying time, it should be optimized by choosing different flocculants, bag types and mesh sizes according to the
actual production in RASs. Ebelinget al.(2006) found that at the optimum combined dosage of alum (mg L1) and polymer (mg L1), the inclined belt filter system increased the dry matter content of the sludge to approximately 13%
solids and reduced the suspended solid and reactive phos- phorus concentrations of the effluent by 95% and 80%, respectively. In addition, significant reductions in total phosphorus (TP), total nitrogen (TN), BOD, and chemical oxygen demand (COD) were observed. Zhang et al.
(2015a) developed a multiway gravity device whose best removal ratio was 58.57% when the HRT reached 20 min.
In addition, the removal efficiency of suspended particles with diameters >60lm could reach more than 90%, but that of suspended particles with diameters <20lm was only 19.5%.
Sedimentation separation not only could remove sus- pended solids and certain small suspended particulate mat- ter but also has been widely used in recirculating aquaculture production pretreatment because of its low cost and low water loss, even though the removal efficiency is not good. Furthermore, as a preliminary treatment, solid–liquid separation not only could effectively remove large suspended particles from the wastewater but also could reduce the pressure on the subsequent filter to improve the efficiency and the entire cycle system, which greatly improves water quality.
Sand filters
Sand filters are effective devices for processing aquaculture water suspended solids; the main material is quartz sand.
These filters are commonly used in RASs and have advan- tages of no pollution, low cost, simple structure, high sus- pended particle removal efficiency, effective backwash recycling, etc.
To solve the difficult problems in the back flushing of traditional pressure-type sand filters after working long time and intercepting a large amount of solid particles, Yu et al. (2014) found that the average removal rate of sus- pended solids of their developed self-cleaning gas strip- ping-type sand filter was 41.31% in recirculation water rearing tilapia; this filter could reduce the COD, ammonia nitrogen and nitrite and remove some particles<30lm in diameter. Timmons and Ebeling (2007) reported that a rapid sand filter could process 94–351 m3day1with a sus- pended solids removal rate of more than 67%, and a parti- cle removal efficiency that was better than the gas stripping-type sand filter, but the rapid filter’s bed head loss reached more than 2.0 m, and its backwash water volume and intensity were big. But the comprehensive gas strip- ping-type sand filter had a slightly better treatment effect.
Their study also showed that during the treatment water volume at 115–700 m3day1, the suspended solids
removal rate of pressure-type sand filter reached 50%, with a head loss of 2–20 m; the removed suspended solids were mainly 30–75lm in diameter. However, the gas stripping- type sand filter had low-head loss and could continuously backwash, allowing the equipment to run continuously and eliminating the backwash water pump and valve of the tra- ditional sand filter. Energy consumption was lower than the sand filter device, and the operation was simple and easy to maintain.
The treatment effect of sand filters is related to filter material of particle size, shape, and the filter layer porosity.
Zhouet al.(2009) reported that when using different parti- cle sizes and particle densities of quartz sand (three layers of two sets in parallel) to process suspended particulate matter in RASs, the removal rate as high as 99.83%, the head loss was little, blowdown effect was good, and the backwash consumption was low. When using sand filter tank to remove suspended solids, the water cost would increase. Therefore, considering the effect of water treat- ment and operation cost, further studies for new filter materials, process, and methods are still needed to realize low cost and high efficiency. Sand filters not only obviously remove suspended particles in water but also can remove bacteria and algae (Sabiriet al.2012). Bomoet al.(2003) reported the use of biological sand filters to remove patho- genic bacteria in fish wastewater and found that several
kinds of bacteria in the water could be removed at the beginning of the test phase, and the removal efficiency remained above 99.9% at last in the test. Chenet al.(2001) designed an internal recycling continuous sand filter after experiments. They evaluated the operation effect of the continuous sand filter under various conditions and per- formed hydraulic calculation and data analysis to form a theoretical calculation formula for the internal recycling continuous sand filter.
Although the sand filter could effectively intercept live aquaculture particles and suspended solids, ammonia nitrogen and COD could also be removed to a certain degree. This filter could meet the requirements for a cer- tain amount of water treated in recirculation aquaculture workshops. Compared with other solid–liquid separation equipments (shown in Table 1), sand filters are low cost and easy to operate, but the medium of sand filter layer needs to frequently be checked to timely eliminate sur- face floating mud and improve the overall water treat- ment system. However, a traditional sand filter tank has its drawbacks, including regular flushing prevention, high flushing pressure, easily hardened filter material, and high maintenance cost. For future application, its defects not only need to be considered but also need to be grad- ually overcome in order to achieve the best treatment effect.
Table 1 Performance comparison of different physical filtration equipments Equipments Filter particle
diameter (lm)
Advantages Disadvantages References
Solid–liquid separation equipment
1–700 Has a better effect on large particulate matters, little floor space,
easy to install and management, low cost, small water loss
Not good at removing little particulate matters
Scott and Allard (1984), Pfeifferet al.(2008), Guet al.(2010), Lee (2010), Lee (2014), Zhanget al.
(2015a) Microscreen
drum filter
>60 Strong applicability, little floor space, easy to maintain, have self-cleaning capacity
Need high-pressure water jet washing, high energy loss, easy to cause large particles of the second broken
Cripps and Bergheim (2000), Vinciet al.(2001), Davidsonet al.(2013), Fernandeset al.(2015) Parabolic
screen filter
>70 Simple structure, easy operation, no power consumption, and low maintenance costs
Low automation, require frequent manual cleaning the screen
Xinet al.(2009)
Sand filter 30–75 Nonpollution, low cost, simple structure, has good
removal effect on particulate matters
Need regular backwash and to be pressure, filter easy to knot
Timmons and Ebeling (2007), Yuet al.(2014)
Foam fractionator
<60 Has a better effect on seawater aquaculture water treatment, low cost
Has a bad effect on freshwater aquaculture water treatment, mechanical
flotation equipments need high energy consumption
Timmons (1994), Schroederet al.(2011), Kanekoet al.(2011)
Protein skimmer
<50 Simple structure, high water treatment
efficiency, better control of water quality
High energy consumption, cause the loss of salt and trace elements in water
Xinet al.(2009), Caoet al.
(2010), Rahmanet al.(2012)
Microscreen drum filters
Microscreen drum filters are alternatives to sand filtration, especially when excessive wastewater is a concern. Due to the overall design and operation of drum screen filters, with few moving parts to ensure a long life with low operating/
maintenance costs, the filtering process is very simple, effi- cient and reliable. Microscreen drum filters are based on the suspended particles in wastewater being bigger than the screen mesh aperture and thus retain suspended particles to achieve solid–liquid separation. In addition, drum rota- tion and backwash clean the screen to ensure its efficient and sustainable removal of solids. Drum rotational energy is basically stable in microscreen drum filter operation; as the screen mesh number increases, the backwash frequency improves, and the power consumption increases with the number of backwashes. Water consumption is also an important index for microscreen drum filter performance evaluation and is proportional to the number of back- washes. Microscreen drum filters are used to remove solid particulate matter>60lm in diameter (Cripps & Bergheim 2000). Microscreen drum filters have strong applicability, take up little floor space, and require convenient mainte- nance, and their treatment effect is closely related to parameters such as hydraulic load rate, mesh pore size, par- ticle concentration and recoil intensity.
In a drum filter, the screen is fixed on a rotating drum frame on a horizontal axis and partially submerged in water; water flows into the drum and radially through the strain cloth, capturing fine particles with a suitable mesh size (Vilbergsson et al. 2016). The filter screen is the main working part of a microscreen drum filter; the mesh number (aperture) directly affects the TSS removal efficiency. The bigger the mesh number, the smaller the aperture size is and the more solid closure, but a higher backwash frequency needed. Peng et al.(2002, 24) used a microscreen drum filter with a mesh aperture of 10–
45lm to remove algae in aquaculture wastewater at a rate of 50–70%. Su et al. (2008) found that the removal rate rapidly increased when the mesh number increased from 150 to 200. The effect was obvious when the screen mesh was 200, the TSS removal rate reached 54.90%
(The relationships of TSS removal efficiency and filter mesh as shown in Figure 1), the washing frequency was 2.1 times h1, the power consumption was 6.902 kW h day1, and the water consumption was 1.68 m3 day1. He (2014) reported that using approxi- mately 260 mesh for the microscreen drum filter could effectively purify water quality and improve rearing den- sity to 143.1% of the original, while using a 420 mesh microscreen drum filter could result in rearing density 161.5% of the original. According to the results of a study by Vinci et al. (2001), for particles 60–100 lm in
diameter, when the inflow water mass concentration was
<5 and >50 mg L1, the removal rate of drum filter was
31–67% and 68–94%, respectively. However, the use of a microscreen drum filter caused big granular particles to crush, producing a large number of small particles and increasing the difficulty of protein separation and biologi- cal filtration.
Chen et al. (2011) improved a traditional microscreen drum filter using the surface microporous filter principle;
they set a certain filtration water level and time and through the reciprocating movement of the screen mesh sieve plate structure and the cleaning system realized real- time backwash of the new continuous microscreen drum filter. Periodic filter cloth replacement could be required in the case of nonstop, and this method could solve the prob- lem of filter clogging, improving the filter capacity. Chen and Cao (2013) established the microfiltration model and analysed the effect and causes of congestion during micro- filtration, including the filtered water, time, and backwash effect. Then, they designed a new continuous water treat- ment microscreen drum filter with real-time backwash to remove solids with less power consumption, Ali (2013) designed and evaluated a microscreen rotating filter driven by an undershot waterwheel for culturing tilapia in a RAS.
The filter only used 18 kW of energy per day, and the effi- ciency of the filter decreased during the first 2 months compared to the last 2 months of the fish growth period, averaging 34.22% during the first 60 days and 52.41% dur- ing the last period. Chen and Chen (2014) made improve- ments to the microscreen drum filter in a RAS by controlling the rotation of the filter bucket and backwash opening through differences water level, it could reduce energy consumption, save labour, and make this filter affordable, easy to operate, and with a low operation cost.
To aid in the identification of an optimal filtration solu- tion in terms of cost and filtration performance, Dolan et al.(2013) expressed that selecting filters with a minimal filtration area was more economical in the short to medium term. As cost savings is achieved by intermittently using
Figure 1 The relationships of TSS removal efficiency and filter mesh.
larger filter backwash, it may take a longer before this filter is more economical to operate than a smaller filter operat- ing in continuous backwashing mode. The most economi- cal unit will be a smaller unit operating in continuous backwashing mode with a filter with moderate cost con- straints. Fernandeset al.(2015) reported that systems with different microscreens (i.e. 100, 60 and 20lm) presented 3.5 times less particles than the control systems, and fine mesh (20lm) treatment groups reaching steady state faster than systems with 60 or 100lm. Summerfelt et al.
(2001) reported an empirical relationship between micro- screen drum filter capture efficiency and the unit’s inlet TSS concentration and estimated that approximately 79%
of the TSSs in the bottom discharge would be captured using microscreen filtration. Davidson and Summerfelt (2005) added settling devices before microscreen drum fil- tration, as the drum filter was found to remove 40–45% of the total mass of TSSs removed daily from the RAS. David- son et al. (2013) found that the microscreen drum filter with 60lm screens removed the majority of TSSs, TP, and TN when comparing the mass of waste removed per kg feed among the three system discharge locations for both diets in low-exchange-water RASs. Khateret al.(2011) indicated that the efficiency of the hydrocyclone and the drum filter to removed settleable and suspended solids in succession ranged from 27.4% to 57.79% and 15.46% to 57.71%, respectively.
In many RASs, microscreen drum filters are used to remove and concentrate suspended solids from process water because these filters require minimal labour and floor space and can handle large volumes of water with little head loss. They produce a separate solid waste stream that can be further concentrated to reduce the quantity and improve the quality of discharged water.
The biggest characteristics of microscreen drum filters are that they have a self-cleaning screen feature to meet the system’s continuous operation requirements and the filter mesh is generally 120–300 mesh; 200 mesh is the princi- pal mesh. To enhance the efficiency of drum filters, recir- culating water pipes from multiple fish ponds could be connected in series, and one filter can be used together for all the ponds. The drawbacks of microscreen drum filters are their requirement of high-pressure water jet washing, large energy consumption, easily damaged sieve silk, high maintenance cost, and easily breaks large parti- cles in the second filtration, resulting in a large number of tiny particles and thereby reducing the fine filtration and biological treatment efficiency (Ni & Zhang 2007).
Before using microscreen drum filters to remove sus- pended particulates, their shortcomings must be com- pletely considered in order to respond well in advance to avoid microfilter incomprehension, which would result in unnecessary economic losses in the productive process.
Parabolic screen filters
Parabolic screen filter (PSF) is separation device that tech- nically derived from a sifted ore. It is mainly used to verti- cally align the inlet and outlet flow direction of the arched fixed-screen surface by centrifugal and gravity force based on the principle, that the particle size is smaller than the diameter of suspended particles discharged from the sieve and greater than the diameter of suspended particles on the screen surface for solid–liquid separation. PSFs are metal mesh structures with high strength, stiffness and load carry- ing capacity; simple structure; easy operation; no power consumption; and low-maintenance cost. They can also separate suspended particles >70lm in diameter particles from RASs (Xinet al.2009), but the automation degree is low, and frequent manual cleaning of the screen is required.
Some researchers have investigated the PSF efficiency of recirculating water treatment. Lianget al.(2011) used PSFs effectively removed 90% of the solid particles from the wastewater in RAS, increasing DO and pH and reducing COD for subsequent water treatment. Zhanget al.(2011) showed that the PSF as the primary filter could effectively remove large suspended particles. Xinet al.(2009) used a stainless steel grid PSF with a 250 lm screen gap in an Epinephelus coioidesRAS. This system could remove more than 80% of solid particles with a particle size >70lm.
Combined with the protein separator, it effectively reduced the water organic content and reduced the unit energy con- sumption of the RAS by 44.35%. Chenet al.(2015) found that the screen gap of PSF should be equal to or slightly smaller than the average size of solid particles in the water, the inlet flow rate and solid particulate removal rate were inversely related, and increasing the installation angle within a reasonable range improved the solid particle removal rate. Danaheret al.(2011) compared a PSF with a cylindro-conical clarifier used in a UV aquaponic system and found that the PSF was able to remove 5.8% of the solids entering it when compared with a 30.8% removal efficiency for clarifier. Through PSF filtration, removing residual feed, excrement and other large solid particles at the beginning of system operation can greatly reduce the load of the entire water treatment system and, in particular, could greatly improve the foam separation and biological filtration processes treatment effect. PSFs are currently widely used in RASs. Although PSFs have advantages, they still require improvement in automation, labour reduction and other aspects. Moreover, the lack of automatic cleaning of the screen surface is a big problem. Even when the rear- ing load is high, the screen surface needs to be manually scrubbed once an hour. During system operation, the seal- ing of PSF must be monitored, and the safety of the screen should be timely checked to prevent damage. In addition,
timely scrubbing and inspecting the screen prevent the mesh from plugging and affect the overall water treatment effect.
Foam separation equipment
For RAS wastewater with small suspended particles, ordi- nary solid–liquid separation equipment only removes some particles, and the foam separation method removes most particles for better water treatment effect (Ni & Zhang 2007). For fine solids removal, foam fractionation is often employed and is also known as air stripping or protein skimming (Timmons 1994; Hussenot 2003). Foam separa- tion involves injecting air into the water so that the surface- active substances in water are absorbed by tiny bubbles, which due to buoyancy rise to the surface of the foam, thus removing dissolved substances and suspended solids in wastewater. Foam separation could effectively remove sea- water (salty) suspended solids and soluble organic matter and reduce the number of bacteria during precision purifi- cation and has O2 addition and decarbonation functions.
However, the effectiveness of freshwater aquaculture in addition to eel farming success is not obvious (Zhenget al.
2005). Air flotation is a common form of foam separation and is currently more commonly used in marine RASs; it has advantages of less site area and easy to use, but its effi- ciency is relatively low, and it has high energy consump- tion. There are some limitations to large-scale aquaculture water treatment applications. The main foam separation equipment has a foam fractionator and protein skimmer.
Foam fractionators
Particles<30lm in diameter accounted for 80–90% of the total number of particles in RASs, and the nutrients carried in these suspended particles included 7–32% TN and 30–
84% TP (Chenet al.1993; Ni & Zhang 2007; Sun & Wu 2008). It is difficult to remove these particles through screening, so foam fractionation technology is needed.
Foam fractionators have a good water purification effect in marine RASs and a good removal effect on small particle suspended solids and dissolved organic matter. But it has some limitations. Due to the freshwater aquaculture lack of electrolytes, low foam formation rate, and poor stability, has a low solid–liquid separation efficiency, so foam frac- tionators are usually not chosen to treat recirculation fresh- water (Zhenget al.2005). Because of their simple process, stable performance, and easy maintenance, they are very suitable for marine recirculating water purification equipment.
Foam fractionators can separate dissolved organic matter and suspended particles from the water circulation so that the BOD and COD are reduced and DO is increased, which can provide favourable conditions for biofilter function.
Lamax (1976) used a biofilter combined with a foam frac- tionator, or a settling pond, or a mechanical filter and found the highest water purification efficiency with the combination of foam separator and biofilter. Timmons (1994) found that the foam fractionator was suitable for the removal of smaller particle suspended solids. Luoet al.
(2008) reported that the foam fractionator removal rates of nitrite and ammonia nitrogen were 42% and 25%, respec- tively in RAS. Yang (2016) reported that the maximum removal rates of TSSs and COD were 30.81% and 25.82%
for the Venturi tube bubble separator, and 25.81% and 22.38% for the scattered gas bubble vent separator, respec- tively. Liuet al.(2006) used an inclined plate settling tank, hydrocyclone and foam fractionator to remove solid parti- cles in self-designed RAS to farm bastard halibut (Par- alichthys olivaceus). The results showed that the tank had the best removal ability for TSSs in farming water, while the foam fractionator removed 53.1% of the most difficult- to-remove particles, of which VSSs accounted for 93.2%.
The efficiency of the foam separator in removing sus- pended particulates is influenced by physical factors (e.g.
gas flow velocity, HRT) and chemical factors (e.g. tempera- ture, pH, salinity; Zheng et al.2005). Zhenget al.(2005) used an aeration foam fractionator to remove suspended solids in mariculture wastewater (31% salinity) and found a removal rate of 36.24–67.05%. Barrutet al.(2013) found that reducing water recirculation efficiency and increasing particulate organic matter concentration improved foam separation efficiency, while increasing airflow velocity reduced separation efficiency. Sun and Wu (2008) reported that the foam fractionator had a good removal effect on suspended particulate matter and dissolved organic matter in RASs and increased the extent to HRT could improve the water treatment effect. Yuet al.(2005) found that the effect of foam fractionator height on removal rate varies with the concentration of organic matter, when the organic matter concentration was 4.32 mg L1, the removal rate obviously increased, the organic matter removal rate was the largest when the gas–liquid ratio was 6. Hu (2012a) found that the change inflow rate had little effect on the water treatment effect of self-designed jet foam fractiona- tor. The micropore aeration foam fractionator increased the ammonia nitrogen, COD and suspended solids removal rate with the increase in gas supply. The water treatment effect of jet foam fractionators was better than that of the microporous aeration foam fractionator under the same treatment conditions and used less energy.
The combined effect of the strong oxidizing and foam separation principle of O3could produce very good results, but excessive use of O3could cause great harm to aquatic products. The research of Schroederet al.(2011) indicated that nonexcessive amounts of O3could be effectively uti- lized to remove nitrite and yellow impurities. A foam
fractionator combined with O3 in short-term use could effectively reduce the propagation of bacteria. However, excessive use could result in large amounts of highly toxic oxidants.
Flotation technology is a kind of foam separation; the main principle is to create highly dispersed microbubbles in RAS application and to use these microbubbles and the water between the tiny particles adhered to the gas–liquid surface to form three-phase particles (gas–liquid–solid) floating on the surface to be removed. Flotation technology has a good removal effect on small particles, viscous mate- rial and ASSs in recirculation water. Liet al.(2007) used a dissolved air flotation device to treat mariculture wastewa- ter (salinity 31&); the COD removal rate reached 70%.
Shan et al. (2013) found that the mechanical flotation device could remove certain TSSs, COD, TN, TP and chroma, which could solve the technical problems of removing suspended particles in brackish-water RASs. In 2015, Shanet al.(2015) found that use self-designed impel- ler flotation device could remove organic matter, COD and TN by foam separation and could removes some TAN and nitrite nitrogen in grouper (Epinephelus sp.) seawater (salinity was 15&) RAS. Mu et al.(2008) found that the removal rate of fine suspended particulate matter peaked when the liquid level was 1 m, the water inflow was 300 L h1, and the pressure was 0.4 MPa using self- designed inflating flotation device.
A RAS could run in industrial wastewater treatment mode, use a flotation comprehensive treatment process and aerator to form microbubbles in the tank bottom, use the bubble diffusion plate to collect protein and other pollu- tants in the surface air bubbles, and then through the sur- face of the dirt collection tank discharge contaminants outside the system. This kind of device makes treatment water endless and has better water treatment effect, making it suitable for RAS application. Compared with traditional jet foam fractionators, the flotation process has advantages of low cost and good comprehensive effect, but its existing problems include high comprehensive energy consumption due to the generation of microbubbles, poor foam flowabil- ity and rapid failure due to decomposition speed separa- tion. In addition, foam fractionators can also reduce trace elements while removing contaminants from aquaculture water (Kanekoet al.2011). Therefore, when foam fraction- ators are applied, change in trace elements need to be con- sidered, and a suitable foam separation mode needs to be chosen according to the production environment in order to confer the biggest advantages of foam separation equip- ment and to achieve the best water treatment effect.
Protein skimmer
A protein skimmer is a set of intensive equipment based on the principle of foam separation that can remove harmful
substances such as suspended matter, protein, ammonia nitrogen and nitrite from a RAS and reduce the load on the biological treatment process. Under the action of jet device, a large number of fine air bubbles are mixed into the water, increasing the contact area of water and air, depleting harmful gases and increasing the DO content. Protein skimmer, because of its simple structure, efficient water treatment and other characteristics, it is advantageous in the aquaculture industry, but they also have disadvantages including high energy consumption and loss of salt and trace elements in water.
Protein skimmers can not only remove organic particles from wastewater but also regulate water quality. Xinet al.
(2009) designed a protein skimmer and used it in a grouper and Cynoglossus semilaevis closed-loop RAS. When the water treatment capacity was 200 m3h1, and impurities smaller than 50lm in diameter were effectively separated.
The removal rates of organic matter and bacteria were 59.84% and 92.97%, respectively, and this skimmer removed some COD, ammonia nitrogen and nitrite. Cao et al.(2010) indicated that protein skimmer played a good role in regulating water quality of a white shrimp (Penaeus vannamei) RAS, and found that the water pH value was maintained at 8.0–8.3, the DO content was 3.775–6.300 mg L1, and the COD content peaked at 14.27 mg L1; this skimmer also had a good effect on ammonia nitrogen and nitrite. Rahman et al.(2012) indi- cated that protein skimming resulted in obviously better water quality in RAS, heterotrophic bacterial abundance and abalone growth and that recirculating abalone culture systems with a protein skimmer housed in an air-condi- tioned, insulated recyclable frozen container may provide a viable alternative to current land-based, flow-through systems.
Protein skimmer can be used in combination with O3, which enters the protein skimmer through the jet device after mixing well with water, and can sterilize, accelerate the decomposition of organic matters and converting ammonia nitrogen and nitrite nitrogen. However, the amount of O3 used should be strictly controlled, and the recycling of O3 and the dispersal of nonozone emissions directly to the atmosphere should be monitored. Songet al.
(2005) utilized a Venturi jet-type protein skimmer in the water treatment of a bastard halibut RAS. When the maxi- mum treated water volume was 50 m3h1and the O3level was 10 mg h1, the suspended solids, protein, ammonia nitrogen and nitrite in the water were significantly removed; DO content increased rapidly; and the disinfec- tion effect was perfect. Park et al.(2011) found that when using 20 g day1 O3 could increase the particle removal rate, significantly reduce the bacterial content, and achieve good water purification in protein skimmers with two dif- ferent doses of O3 to culture black sea bream
(Sparus macrocephalus) in a marine RAS. Attramadalet al.
(2012) researched the effects of moderate ozonation or high-intensity UV irradiation combined with protein skim- mer on the microbial environment in a RAS for marine lar- vae. This study found that the O3 system had a more mature and stable microbial community than did the UV system, but the O3system had a low disinfection efficiency with moderate ozonation. Schneider et al. (2012) found that a protein skimmer used in a flatfish RAS could increase profit through fish performance and improved fish welfare due to better water quality.
A protein skimmer is based on the principle that the con- tact surface formed between air and water has a certain sur- face tension and directly adsorbs organic substances from the water for the rapid removal of harmful organic particles and to prevent the deterioration of water quality. A large number of microbubbles formed in the jet melt in the water, with the exchange and removal of harmful gases while reducing the hardness of carbonate and phosphate;
the pH was stable (Caoet al.2010). In recent studies, the use of protein skimmers in combination with O3has been the most effective in purifying water, and the optimum amount of O3 should be used to achieve the best water purification effect when applying protein skimmers.
Physical filtration equipments conclusion
By reading the relevant literature on physical filtration equipment, from performance, applications, advantages and disadvantages and so on, we provide a comprehensive analysis of the physical filtration equipment. Then, we compare the performance of the physical filtration equip- ment as shown in Table 1.
From Table 1, it can see that different pieces of physical filtration equipment could remove solid particles of differ- ent sizes. And, a combination of these pieces of equipments could be used to remove most the solid particles>30lm in wastewater. In addition, we also summarized the advan- tages and disadvantages of physical filtration equipments and make comparisons to provide a reference for recircu- lating aquaculture workers, so that they could utilize this equipment as reasonably as possible.
As environmental regulations become more rigorous, environmentally sound waste management and disposal practices are increasingly more important in RASs (Ebeling et al. 2006). Suspended solids removal should meet the requirements of small particle processing capacity to avoid biological clogging and reduce backwash energy consump- tion as much as possible to improve the hydraulic load rate and reduce equipment investment and operating costs.
Sometimes, the effect of removing solid particles through a combination of several devices is better than that using one device alone. Application of the optimal combination of
related technologies to remove different size particles, for example, combine LPH with microscreen drum filter to remove bigger than 60 lm particles, and can also combine with protein skimmer to remove bigger than 30 lm sus- pended solids; in marine RASs, in order to improve the fine suspended solids removal efficiency, drum filters or equiva- lent devices are often combined with foam fractionation systems. Davis and Arnold (1998) described using a 100 lm microscreen filter, a foam fractionator and a set- tling chamber to remove particles. To date, it is unclear how to control and remove different solids fractions in a cost-effective and treatment efficient way. The physical fil- tration equipments of RASs should reduce costs and improve the automation and intelligence degree in future development, and should allow the effective removal of solid particles while avoiding generation of other by-pro- ducts (e.g. ammonia nitrogen, nitrite, organics, etc.) and should increase the load after physical filtration while removing harmful organic matter, ammonia nitrogen and nitrite (Sun & Wu 2008; Zhou et al. 2009; Hu 2012a), thereby enhancing the water purification effect and the effi- ciency of the whole water treatment system.
Biological filtration equipment
Biological purification is the core of RASs and includes the selection of biological medium and the culture of biological membrane. Ammonia nitrogen and nitrite nitrogen are the main metabolic wastes that are produced by residual feeds and excrement from RASs (Zhan & Liu 2013; Song et al.
2014). Biological purification is also a biological removal nitrogen process, which is a nitrification reaction that oxi- dizes NH4+
-N to NOx-N (NO3-N or NO2-N) and a denitri- fication reaction that reduces NOx-N to N2. Nitrification is the conversion of NH4+
-N to NOx-N under aerobic condi- tion, which is achieved by autotrophic bacteria; and deni- trification is the process by which denitrifying bacteria reduce NOx-N to N2under anaerobic condition, which is achieved by heterotrophic bacteria (Guoet al.2005). Aero- bic or anaerobic biological filtration through direct contact with microorganisms and wastewater uses decomposition to absorb TAN and nitrite nitrogen to improve water qual- ity. Biological filter processing power mainly depends on whether the filter can carry bacteria quantity. The number of bacteria carried by the biofilter determines its water treatment capacity. In other words, a strong water purifica- tion capacity requires a large specific area and large bacte- rial communities. The specific surface area indicates the surface required for biofilm growth as well as the homoge- neous water flow related to dead zones and channels in the system. The TAN and nonionic ammonia in the water should not exceed 1 and 0.05 mg L1, respectively, and in cold aquaculture systems, they should be maintained at
0.1–0.5 and 0.1–0.3 mg L1, respectively (Losordo et al.
1998). To maintain the water quality environment, redun- dant TAN and nonionic ammonia must be removed. The medium is the core unity in a biofilter and is usually elastic packing, stereoscopic net structure medium, biology ball, biological ceramsite, etc. Commonly used biological filters include fluidized-bed biofilters, moving-bed biofilters, fixed-bed biofilters, trickling filters, RBCs, and bead biofilters.
Fluidized sand biofilter
The fluidized sand biofilters (FSBs) are fluidized-bed reac- tors that use sand as a carrier and keep it in suspension or fluidized by the upward water flow (Sanchez & Matsumoto 2012). The fluidized bed offers a novel option for denitrifi- cation treatment which widely used for RAS nitrification (Summerfelt 2006). Fluidized biofilters provide very high specific surface area which could provide aerobic organisms with sufficient surface area to create hypoxic conditions, while still allowing heterotrophic denitrification growth in the supplemental area. Summerfelt (2006) summarized the most important aspects of sand selection and estimates of nitrification rate, ammonia removal efficiency, CO2 pro- duction, and O2 consumption across FSBs under various conditions. Conventional FSBs have been widely adopted in North America, especially in RASs that must reliably maintain perfect water quality to produce species such as salmon smolt (Oncorhynchus), arctic char (Salvelinus leuco- maenis), rainbow trout, endangered fish, and tropical or ornamental fish. A specific application of liquid–solid two- phase FSBs is as silver sand act biofilm carriers; the sand density is approximately 2.65 g cm3, and the diameter is generally 0.15–1.19 mm (Timmons & Ebeling 2007). This kind of filter has the advantages of a compact structure and low cost but requires huge energy consumption during start-up, and so on.
To manage the problem of dramatic increase in bed expansion due to biofilm formation, the study of Davidson et al. (2008)indicated that FSBs could effectively remove 86–88% TAN, 66–82% cBOD5, and 1–2 log10total coliform bacteria from high volume intensive aquaculture effluents and were capable of low-level removal of phosphorous and TSS. This study also demonstrated that a smaller sand size (0.11 mm) resulted in increased nitrification and removal of cBOD5, as compared to a larger sand (0.19 mm), partic- ularly bed growth was controlled using a biofilm shearing method. Summerfelt (2006) introduced five different water mechanisms in the traditional design and management of FSBs: the selection criteria for filter particle size, the design and selection of fluidized bed, common problems during operation, a smaller sand particle size producing a more strongly attached microbial membrane and better
wastewater treatment effect, and the more uniform distri- bution of the filter media after expansion of the bed to equal to twice bigger than the bed diameter. Zhang et al.
(2015b) found that although the filming time was longer than that for inoculating raw materials under low-tempera- ture conditions in a small sturgeon RAS, the FSB had a bet- ter nitrification performance and better effect than the average biological filters.
The North American grow-out systems described by Summerfelt et al.(2004a) primarily used FSBs, which are relatively inexpensive and have a high specific surface area (8000–12 000 m2m3). These biofilters remove TAN and oxidize nitrite efficiently with typical outflow concentra- tions of TAN between 0.1 and 0.5 mg L1 and of nitrite
<0.1–0.3 mg L1. Christianson and Summerfelt (2014)
presented to use an FSB for sulphur-based autotrophic denitrification in aquaculture effluent. These authors used three elemental sulphur materials (described as ‘flakes’,
‘grains’ and ‘powder’); the finest product was an elemental sulphur powder with an effective size of 0.08 mm that pro- vided a greater bed specific surface area than that provided by the other materials. Zhan and Liu (2013) used an FSB and immobilized cell technology to rear rainbow trout and common carp in a RAS; the removal efficiencies of TAN and nitrite nitrogen were 80–95% and 80%, respectively, and the amount of water used was reduced by 80–90%.
The structure of FSBs has undergone transformation from liquid–solid two-phase to gas–liquid–solid three- phase. Three-phase fluidized beds have been targeted to solve the problems of two-phase fluidized-bed filter media in uneven distribution to ensure efficient and stable opera- tion of the filter. The three-phase FSBs that have been used in aquaculture can be briefly described as follows. In aero- bic biofilters, O2is consumed as it penetrates the biofilm until it reaches values suggesting hypoxic or anaerobic con- ditions. Thus, two biofilm layers, an aerobic layer that could potentially perform nitrification and an anaerobic or anoxic layer that could perform denitrification, could be developed depending on the COD concentration in the external fluid and the biofilm thickness. Tsukuda et al.
(2015) used triplicate FSBs for rainbow trout and Atlantic salmon reared in a RAS; they operated at an HRT of 15 min and a hydraulic loading rate of 188 L m2per min, and found that during the biofilter study period, nitrate reduction was consistently observed although nitrite nitro- gen and TAN concentrations slightly increased (11% and 13% increases, respectively). The nitrate removal efficiency was closely related to the COD-to-nitrate ratio. Sanchez and Matsumoto (2012) used a three-phase fluidized bed of activated carbon filter at an HRT of 11.9 min for Nile tila- pia cultured in a RAS and found that the average removal efficiencies of BOD, COD, TP, TAN and TN were 47%, 77%, 38%, 27% and 24%, respectively. For space-efficient
FSBs to be used as denitrification reactors, an endogenous carbon source could evaluate system parameters (e.g. influ- ent DO and carbon-to-nitrogen ratios) most effectively.
Timmonset al. (2000) improved the structure of FSBs by inventing and successfully promoting a more expand- able sand bed filter and cyclonic sand biofilters (CycloBio;
shown in the Fig. 2). Summerfeltet al.(2004b) improved CycloBio, bringing a reverse cone and tangential water dis- tribution technology so that the expansion of the sand bed could be followed by rotation, decreasing start-up energy consumption. The expansion of the pure sand bed could be increased by 60–80%; the uniformity of expansion was bet- ter when the height of the sand bed was twice the diameter of the filter, which greatly increased the water purification capacity. Liuet al.(2015b) found that the best combination of structural parameters was a cone height and diameter of 60 and 165 mm, respectively, and a slot width of 1.0 mm;
the structure of the optimized CycloBio was more reasonable.
Some of the main advantages of the fluidized-bed filters described by Timmons and Ebeling (2007) included the facts that FSBs are very economical to build from commer- cially available materials, their original filter media have a very high specific surface area at low cost, and they could be field built using a variety of proven methods. FSBs pro- vide an excellent environment for the growth of nitrifying bacteria. On the one hand, the media increase the surface area to which the bacteria can attach (and almost all bacte- ria could attach to the filter); on the other hand, the unsteadiness environment could release bacteria from the filler surface and achieve self-purification. Although the ammonia nitrogen conversion capacity and efficiency of FSBs are very high, but when it uses FSBs to purify farming wastewater, it must full consider the problems of large energy consumption, loss of media and difficulty for the start-up to form a fluid. In the future, it needs try to over- come the disadvantages in order to achieve the maximum cost-effectiveness of FSBs.
Moving-bed biofilm reactors
Moving-bed biofilm reactors (MBBRs) were developed by Kaldnes M. with the SINTEF research institute in collabo- ration in Norway in the late 1980s (Hem et al. 1994).
MBBRs have been widely applied for water treatment and were recently established as a popular and reliable biofilter type in RASs. The medium is the core of MBBR: it is an important place for biofilm to attach and grow organic matter and the basic means to enhance mass transfer and improve water conditions in the reactor, and it is also a fundamental way to increase load. Medium performance has a direct impact on biofilm culturing, the amount of biomass in the reactor and the treatment effect. The media used by MBBRs mostly are polyethylene, polypropylene plastic or polyurethane. The media form a fluidized state under waterflow and airflow agitation (usually aeration in aerobic reactors and mechanical agitation in anaerobic reactors), which is a biological treatment mode that combi- nes activated sludge with suspension growth and biofilm with attached growth. MBBRs are distinguished by com- bining several advantages over alternative biofilter types, such as a volume-effective relation between active surface area and reactor volume, stable and low-maintenance oper- ation, no need for periodic backwashing and no susceptibil- ity to clogging. Recently, some researchers studied water treatment of MBBRs in RASs as follows.
Some researchers have compared the performance between moving-bed biofilters and other biofilters and found that performance was better in moving-bed biofil- ters. Drengstig et al. (2011) reported that MBBRs were preferable over fixed-bed biofilters and trickling filters, and the same conclusions were drawn by Dalsgaard et al.
(2013). MBBRs are more compact, use media with a larger specific surface area (500–800 m2 m3), and are easier to operate and maintain than fixed-bed biofilters and do not require backwashing. Michaudet al.(2014) suggested that the plastic moving beds had less nitrification than the static mineral beds did, and the increased C/N ratio led to a change in bacterial community structure, specifically, reduced taxa richness and diversity indices, and to a posi- tive selection of Gammaproteobacteria.
In recent years, studies have focused on improving MBBR water purification efficiency. Lou (2007) overcame the disadvantage of the large motion dead angle of tradi- tional MBBRs and successfully developed the Improved Moving Bed Bio Reactor (IMBBR), and indicated that when the reactor length ratio was approximately 0.5, the rising area and down area ratio was 0.67, the filling rate was approximately 50%, and the wastewater treatment effi- ciency of IMBBR was the highest. M€uller-Belecke et al.
(2013) designed a self-cleaning inherent gas denitrification reactor (SID reactor), which is an enclosed MBBR driven
Figure 2 Cyclonic sand biofilters.
by recirculation of inherent gas and found that SID reactor provided satisfying long-term denitrification with minimal energy demand for discontinuous inherent gas recircula- tion and for the production of species sensible to nitrate.
Pfeiffer and Wills (2011) evaluated three types of media (K1 Kaldnes, MB3, and AMB) in hatchery-scale MBBRs in a marine RAS and indicated that the per cent TAN removal rates for MB3 media were the highest at both low and high feed load rates, averaging 12.3% and 14.4%, respectively.
The volumetric TAN removal rates for the respective feed load rates and TAN removal efficiency results were very obvious. Summerfeltet al.(2015) investigated Atlantic sal- mon smolt producers using soft water make-up sources should aim for 70 mg L1alkalinity considering the rela- tively low loss of inorganic carbon compared to 200 mg L1alkalinity, the increased pH stability and the reduced TAN concentration, compared to when using 10 mg L1alkalinity in semi-commercial water RASs oper- ated with MBBRs. Songet al.(2012) researched the nitrifi- cation function of MBBR in a RAS, and showed that when the HRT was 10 min, the volumetric TAN removal rate was up to 110.19 g (m3day)1, and the nitrification effi- ciency was the highest.
An MBBR is an attached growth biological treatment process based on a continuously operating, nonclogging biofilm reactor with low-head loss, a high specific biofilm surface area, no need for periodic backwashing and no sus- ceptibility to clogging. Future research will need to evaluate the existing and new structural plastic media to optimize the potential of nitrification and operation, not only for toroid biofilters but also for other types of moving-bed biofilters and reactors. Shear forces act on the outer layers of the biofilm growing in the carrier material of MBBRs, maintaining it at a steady level by controlling excessive bio- film formation and consequently risking an increased sys- tem particle load. MBBRs can be used alone, in a multilevel series or combined with other biological treatment process and have a good removal rate of organic matter, nitrogen, phosphorus, etc. MBBRs have a good development and application prospect.
Fixed-bed biofilm reactors
The recirculating aquaculture industry has tended to use fixed-bed biofilm reactors (FBBRs) rather than sus- pended growth systems. Biofilms could be seen as a bac- terial habitat that withstands a wide range of flow and quality systems while maintaining its inherent ability to process wastes. In a fixed-film biological process, dis- solved or colloidal wastes are transported by diffusion into the biofilm, which coats a filter medium, and rock, shells, sand and plastic are usually used to support these bacterial films in FBBR.
In biological purification, medium, temperature, DO and TAN contents affect FBBR digestibility. Song et al.
(2010) reported that the TAN removal efficiency of the bamboo hollow biochemical ball filter in four kinds of media (e.g. bamboo hollow beads, medical stone, ceramsite and biological ball) was the best in a laboratory simulation test to research the biofilm culture process of FBBRs and the efficiency of mariculture wastewater treatment., paving the way for the process design and media selection of an optimized filter. Zhu and Chen (2002) found that if DO limited microbial growth in experiments, the nitrification performance of FBBR did not change significantly with temperature; they considered that if the limiting factor was TAN concentration in the inflow substrate, the sensitivity of temperature to nitrification performance was much greater than that of DO.
The effects of water treatment differ with different biofil- ters in different media, experimental conditions and envi- ronments. The study of Choi et al. (2012) would help determine the practical significance of MBBR and FBBR removal of bio-P and denitrification from wastewater. They indicated that the amount of all nutrients (total COD, fil- tered COD, BOD5, acetate, PO4-P, NO3-N) removed by FBBRs was higher than that removed by MBBR. Suhr and Pedersen (2010) found that the FBBRs with high porosity and moderate specific surface area (200 m2m3) were more stable in response to changes and removed more sur- face-specific TAN (0.46 g (m2 day)1) compared to MBBRs (0.27 g (m3day)1) in submerged FBBRs and MBBRs from a commercial outdoor RAS. Pedersen et al.
(2015) investigated biofilter-specific nitrification perfor- mance in FBBRs and MBBRs under identical steady-state conditions and during a water treatment event in which 50 mg L1 hydrogen peroxide was applied in an experi- mental RAS. The FBBRs were found to be more robust, protecting nitrifying bacteria against the effects of a mild disinfectant. Fernandes et al. (2017) reported that both FBBRs and MBBRs types tested removed filtered BOD5
(highest in FBBRs) and particulate BOD5 (highest in MBBRs) in an experimental RAS. Neither organic matter nor microparticle dynamics in the system seemed to affect the nitrification performance of the filters tested under the measured ranges.
The main differences between FBBRs and MBBRs are the induced motion of the media-mechanically or by aeration - and the self-cleaning ability of MBBRs. FBBRs have draw- backs, including bacterial membranes and adsorbents that grow on the filter media, may clog the gaps between media, produce a directed waterflow and cause some parts of the filter to lack O2. Fixed-bed biofilters should be backwashed at least once a week to remove particulate matter, which aids in controlling heterotrophic bacteria growth and enhances nitrification processes (Svendsen et al. 2008;
