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The mud that accumulates at the bottom of the primary clarifier is also called primary mud. Typically, a portion of the activated sludge is returned to the system, called activated sludge return, and the remainder is removed at the bottom of a secondary clarifier, called excess sludge, or secondary sludge, or waste activated sludge (WAS). Exposure to biodegradable substances previously inaccessible to microorganisms and changes in the composition of difficult-to-degrade compounds lead to increased biodegradability.

Aerobic thermophilic co-treatment: The process involves two different phases: a biological wastewater treatment and a thermophilic aerobic digestion of the resulting sludge. The solubilized sludge is then returned to the aeration tank in the wastewater treatment step for further decomposition. SRT = V/Q where V is the working volume of the reactor (ml), Q is the sludge flow or loading rate (ml/day).

The inhibitory concentration of a substance depends on many variations, including pH, organic load, temperature, hydraulic loading, the presence of other materials, and the ratio of toxicant concentration to biomass concentration. Anaerobic thermophilic pretreatment: (a) temperature sophase system AD;. b) single-stage MAD; (c) Thermophilic AD processes [17].

Figure 2. Collection of pretreatment techniques and sludge types [2]. The pie-chart corresponds to the number of times each sludge type occurs in combination with a pretreatment

Ultrasonic pretreatment technique

In addition, sludge particle size decreases very quickly due to the increase in US duration, especially in the initial period of ultrasonic process, and much faster than COD release in the water phase. The CST of sludge decreased at lower PUS and US duration because the flocs did not reduce their sizes, but with an increase in US duration at the same PUS, the CST value increased [71]. 76] found that an increase in US doses (0-above 2000 kJ/L) led to a decrease in CST (from 53s to below 10s), implying that ultrasonic treatment of WAS improved dewaterability.

84], the sludge dewatering capacity will increase when the sludge decomposition rate (DDCOD) is 2-5%, because the floc structure has a limited change at DDCOD less than 2%, the number of fine particles in bound water increases at DDCOD 6-7%, however, sludge particle size is significantly reduced at DDCOD above 7%. On the other hand, the sludge dewatering capacity gradually decreased with an increase in ES density, duration of US [83, 86], cell lysis and release of biopolymers from extracellular polymeric substances (EPS) and bacteria into the aqueous phase [15, 85], and a decrease in free water in stool [85]. Sludge settleability varied with increasing ES (increased after the first hour and then decreased), with the optimal ES for improving WAS settleability being 1000 kJ/kgTS [74]. The settleability of WAS improved at ES less than 1000 kJ/kgTS due to slight floc disruption; on the contrary, settleability deteriorated at ES of more than 5000 kJ/kgTS [74] due to complete breakdown of flakes and an increase in EPS concentration in the liquid phase.

The turbidity of the sludge increased due to the increase in ES and particle size reduction during disintegration [75]. In addition, the flocs and cell wall will be completely degraded with the increase in US duration [73, 87]: after 60 min of sonication [73]. The increase in proteins decreased after longer US duration, while polysaccharide and DNA concentrations decreased after 20 min of sonication [86].

In addition, organic nitrogen and ammonia concentrations in sludge samples increased due to increased ES and TS content of WAS. Bacterial cells disintegrated and intracellular organic nitrogen was released into the aqueous phase, which was then hydrolyzed to ammonia, resulting in increased ammonia-N concentration [91]. In the second stage, some cells were exposed and damaged by ultrasonic cavitation, leading to the release of intracellular organic matter, further increase of SCOD and significant decrease of SOUR.

Due to the heterogeneity of the sludge and differences in the external resistance of many species of zoogloe and bacteria, activation and inactivation were effective at the same time, and the overall efficiency was influenced by different ultrasonic parameters.

THERMAL PRETREATMENT

CHEMICAL PRETREATMENT

Oxidation

On the other hand, the disadvantages of thermal pretreatment are to increase the largely soluble inert fraction and final effluent color [110], as well as ammonia inhibition in the main digester due to increased performance [111]. 121], biogas production increased by 80% at 0.1gO3/gCOD ozonation; higher ozone doses, although still positive, were found to have a less pronounced effect. Biodegradation was also found to increase with ozone dose up to 0.2 gO3/gSS, but further increase in ozone dose did not improve biodegradation [122].

Better performance and lower ozone consumption were achieved during post-treatment and recycling in the digester [126].

Alkali treatments

117] found about 60% of SCOD generated due to ozonation to be biodegradable at the early stage of ozonation, while the remaining soluble organic matter was refractory. COD in solution than the individual pretreatments, due to the complementary effects of hydroxyl anion reactions (solubilizing extracellular polymer matrix) and mechanical shear force (disrupting flocs and cells). 133] investigated the hydrolysis rate of alkaline, ultrasound, chemi-ultrasound and simultaneous ultrasound and alkaline pretreatment on WAS (1% of TS fight at ambient temperature).

The authors indicated that the initial hydrolysis rate of the third approach was the highest (211.9 mg/L*min). The second approach was more effective in SCOD release and soluble organic nitrogen compared to the first, but being closed to the third. SCOD was used as an indicator to evaluate the effectiveness of different combinations in the pretreatment stage as well as in the subsequent AD.

Low NaOH dosage (100 g/kg dry solid), short duration of NaOH treatment (30 min) and low ultrasonic specific energy (7500 kJ/kg dry solid) were found to be suitable for sludge degradation. In the subsequent AD, the degradation efficiency of organic matter was increased from 38.0% to 50.7%, which was much higher than with ultrasound (42.5%) or with NaOH pretreatment (43.5%) at the same retention time . The higher efficiency of chemi-ultrasound is due to the combination effects of hydromechanical shear force and OH radical reaction.

Pretreatments improved the subsequent anaerobic digestibility of WAS with significantly high TS and VS destruction and biogas production, but no methane improvement in the biogas. The hydrolysis rate of chemical-ultrasonic treated sludge was higher than that of ultrasonically treated and untreated sludge. The authors found that the increase in solubilization (SCOD/TCOD) in individual pretreatments was limited (50%); however, it reached 70% in combined method, indicating that high sludge pH levels played a crucial role in improving the subsequent US pretreatment efficiency.

However, approximately 20 % higher soluble N concentration found in the reactor after anaerobic digestion will be an additional burden in the subsequent nitrogen removal system.

Acidic Pretreatment

In addition, the pretreated sludge (pH 9 + ES of 7500 kJ/kgTS) was fed to a 3 L anaerobic batch reactor for sequencing after 70 days of control operation. It was expected that the combination of acidic and mild pretreatment techniques with ultrasound (acid-ultrasonic pretreatment) would disrupt the flake structures and release organics into the liquid phase and consequently reduce the overall consumption of energy and chemicals. In addition, it was expected that the physical characteristics (such as dewaterability and turbidity) of the pretreated sludge would be much improved compared to ultrasonic pretreatment alone.

COMPARISON OF PRETREATMENT TECHNIQUES

The primary requirement of a pretreatment is the effectiveness of digestion before digestion; however, acidic pretreatment was unable to effectively dissolve organic matter. However, the lower the pH value, the worse the digestion was due to the antagonistic effect of the acid in the ultrasound pretreatment. From Table 3 it can be concluded that the digestibility may depend on the ability of the pre-treatment rather than on the ES to break the hairs (mechanical or chemical effect).

The global improvement in TSS reduction (after pretreatment + digestion) increased with an increase in TSS solubilization (after pretreatment only) regardless of the type of treatment (under both aerobic and anaerobic conditions) (Figure 10). In terms of economic efficiency, based on the utilization costs with laboratory-scale devices (low energetic performances), Salsabil et al. 6] showed that the application of a pretreatment before AD always led to the cost reduction compared to the control and 8% for sonication, high thermal treatment (90°C and autoclave) and ozonation and low thermal treatment, respectively.

141] considered a 250,000 PE virtual WWTP to compare mixed pellet milling, ozonation, centrifuge-lysis and sonication, providing several classifications of pretreatments. Increasing polymer demand for dewatering: ozonation > sonication > mixed ball mill > lysis centrifuge. Increase in soluble COD and ammonia concentrations in supernatant after dewatering: ozonation > mixed ball mill > lysis centrifuge > sonication.

However, thermal pretreatment also increases particle size by particle agglomeration due to the formation of chemical bonds under high temperature [139]. The effects of pretreatment on WAS were also compared in terms of pretreatment mechanisms, energy inputs, and sludge characteristics. As shown in Table 4, the pretreatment mechanism has been claimed to be more important than the energy input [6].

Regarding sludge characteristics (primary sludge, WAS, or mixed sludge), for example, the effect of pretreatment on WAS depends on the initial biodegradability of the sludge, which in turn depends on the age of the wastewater treatment process sludge. black.

Figure 9. Methane production vs. SCOD for different pretreatment techniques [139].

CONCLUSIONS

Combined thermophilic aerobic process and conventional anaerobic digestion: effect on sludge biodegradation and methane production, Bioresour. Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-phase mesophilic and thermophilic digestion of sewage sludge, water residue. Mesophilic anaerobic digestion of waste activated sludge: influence of solids retention time in the wastewater treatment process, Process Biochem.

Effects of different pre-treatments on enhanced anaerobic digestion with waste activated sludge, Journal of Bioscience and Bioengineering. Pretreatment Mechanisms During Thermophilic-Mesophilic Temperature Staged Anaerobic Digestion of Primary Sludge, Water Res. A new process configuration for anaerobic digestion of source-sorted domestic waste using hyperthermophilic pretreatment, Bioteknol.

Biphasic Anaerobic Digestion of Waste Activated Sludge: Effect of Extreme Thermophilic Preference, Ind. Anaerobic metabolism: impact of future greenhouse gas emission reduction policies on methane production and use, Water Sci. Ultrasonic Conditioning of Waste Activated Sludge for Enhanced Aerobic Degradation, Proceedings of the IWA Specialized Conference - Sustainable Sludge Management: State of the Art, Challenges and Prospects, 29-31 May, Moscow, Russia, 2006.

Effects of thermal treatments on five different waste activated sludge samples solubility, physical properties and anaerobic digestion, Chem. Combined Ozone Pretreatment and Anaerobic Digestion for Reducing Biological Sludge Production in Wastewater Treatment, Ozone-Sci. Anaerobic digestion of waste activated sludge combined with ozone post-treatment and recycling, Water Sci.

Anaerobic digestibility of ultrasound and chemically pretreated waste activated sludge, A thesis of the master's program, 2008. A hydrolysis/thickening/filtration process for the treatment of waste activated sludge, Water science and technology. Effect of ultrasonic, thermal and ozone pretreatments on activated sludge solubility and anaerobic biodegradability, Chem.