Several known conventional methods for removal of nitrates in water are chemical adsorption and denitrification, ion exchange, electrodialysis, capacitive deionization (CDI), and reverse osmosis (Naderi et al., 2012). Having the pros and cons of application, and their efficiency and economic feasibility in nitrate removal from water can be compared.
Autotrophic and heterotrophic processes of biological denitrification can be of the fixed-film (attached growth) or suspended growth mode. In fixed-film denitrification process, the organisms are bound to an inert support media. In spite of a number of applicable media, the development of biofilm can be only successful when the available surface area is maximized. Some instances include packed bed
Location Nitrate / Nitrite Total Phosphorus (TP)
Classification of Pollution (NWQSM) / Exceeded Threshold (DOE Standards)
Reference
Pahang River Kuantan River Belat River
2.64 mg/L (Nitrate) 1.64 mg/L (Nitrate) 1.64 mg/L (Nitrate)
0.79 mg/L 0.93 mg/L 2.24 mg/L
NWQSM Class II, IV (NO -) DOE Standard A & B Galing River (TP)
Kozaki et al., 2019
Galing River 4.71 mg/L (Nitrate) 13.65 mg/L Johor Bahru
sewage treatment plant Kuala Lumpur sewage treatment plant
22.0 mg/L (Nitrate) 18.2 mg/L (Nitrite)
8.89 mg/L (Nitrate) 19.4 mg/L (Nitrite)
10.0 mg/L
9.0 mg/L
DOE Standard A & B Nitrate: Johor Bahru plant.
Nitrite: Johor Bahru, Kuala Lumpur and Penang plants.
TP: Penang plant.
Sabeen et al., 2018
Penang sewage 15.4 mg/L (Nitrate) 12.0 mg/L
reactors, fluidized bed reactors, as well as biofilters consisting of activated carbon, anthracite, calcium carbonate, sand or sulfur (Mohsenipour et al., 2014).
Notwithstanding, they usually have slow reaction rates, high system complexity and thus with high needs of monitoring. Such methods are also sensitive to environmental conditions by elevated levels of nitrate, for instance, bacterial contamination and pH (Mohsenipour et al., 2014).
On the other hand, chemical and physicochemical methods are rather commonly implemented to treat nitrates in contaminated water, and one of which is reverse osmosis. The feed water is applied with high pressure and forced through semi-permeable membranes, which ensure the filtration of all unwanted impurities from the passing feed water. This process usually does not focus on the elimination of targeted contaminants, thus it performs well in removing a number of contaminants such as nitrates. Depending on the original water temperature, quality and system pressure, the nitrate removal efficiency can approach 95% in reverse osmosis systems (Singh et al., 2019). Nonetheless, it has the risks of compaction and deterioration with time and fouling that require pre-treatment, high energy demand and costs. Besides, it is inefficient with very low quality of feed water (Mohsenipour et al., 2014).
In addition, electrochemical reactions of redox have also been widely applied in the nitrate treatment using the electrically switched ion exchange (ESIX), electrodialysis and metallic zero valent methods. The operating principle of ESIX
method involves local, transient formation of ionic groups with high affinity for ionic solutes on the surface of functionalized electrodes. In nitrate elimination, ESIX has been making use of conductive polymer active electrodes (Palko et al., 2018). However, practically, the ion exchange capacity of some electroactive ion exchange materials (EIXMs) in the redox reaction is less than their empirical predictions. This is because of their compact structure in bulk forms which makes the inner layer unable to be thoroughly employed. Besides, some inorganic metal hexacyanoferrates (MHCFs) have undesirable levels of conductivity and film- forming property. On the other hand, some organic conducting polymers (CPs) may suffer from the loss of active materials and over-oxidative degradation through the long- term charging/discharging, and have poor stability due to the shrinkage, expansion, breaking or cracks in the process of doping/de-doping (Du et al., 2016).
Moreover, electrodialysis method consumes high energy with its complicated system and thereby assures with high operation costs, apart from the needs to maintain in rural areas. With concentrations of ion that are too low, it could result in low ion removal efficiencies (Choi et al., 2015). Furthermore, metallic zero valent method may have low selectivity for the target contaminants under oxic conditions, apart from limited efficacy for some refractory contaminant treatment (Mohsenipour et al., 2014).
Capacitive deionization (CDI) is another chemical operating principle of nitrate treatment which utilizes electrostatic interactions to bind ionic contaminants from solution onto electrodes with high surface areas, which are then released into a concentrated waste stream. On the other hand, the purified water is flushed with
more feed water from the cell to finish a cycle. Optionally, it can be used in combination with ion selective membranes to enhance charge efficiency. The adsorption of ions can be owing to the applied potential difference to electrodes, or without applied potentials by the action of native chemical charges on electrode surfaces. The latter is known as inverted CDI, albeit the function of nitrate affinity still has not been adequately studied (Palko et al., 2018). CDI often poses some disadvantages such as membrane fouling, scaling, and high energy consumption (Pastushok et al., 2019).
2.4.2 Phosphate
The conventional methods for phosphate elimination could be based on biological, physical-chemical, chemical or biological-chemical principles. Comparatively, they have their own pros and cons of application. Contemporarily, a number of chemical and physical-chemical methods are available to eliminate phosphate from wastewaters, which include the implementation of magnetic field. The phosphates are incorporated with a reagent in insoluble compounds, on which magnetic material is added to create a magnetic field that isolates phosphate-containing sediment. Besides, the electric coagulation and floatation treatment makes use of electrodes of both aluminum and iron/steel and ensure complete phosphate removal. In addition, crystallization method involves the growth of phosphate crystals in wastewaters at crystallization centers which are then removed from the system. Crystallization takes place in the suspended sludge or on the filter.
Nonetheless, none of the mentioned methods has been widely practiced because of
the high implementation and chemical costs, their relative complexity in the pre- treatment processes, and the possibility of secondary pollution incurred by coagulants (Li et al., 2016).
Biological methods of phosphate removal introduce phosphorus in the cellular components of microbes applied in the biological water treatment process.
In water treatment facilities, the configuration of biological suspended growth process has been periodically adjusted to achieve eradication of biological phosphorous. The main benefits of biological methods of phosphate treatment are minimization of chemical costs and lower sludge production than chemical precipitation. Very often, it does not stably reduce phosphate from the drain fluid to the permitted concentration threshold (Ruzhitskaya and Gogina, 2017). Making use of activated sludge, it eliminates only at most 40% of phosphorus, while it can still be upped to 50% by utilizing more sludge. Nonetheless this is inadequate to cater for 95% of phosphates removal to reach the levels of maximum permissible concentration (MPC) (Ruzhitskaya and Gogina, 2017).