Removal of total ammonia nitrogen (TAN), nitrate and total organic carbon (TOC) from aquaculture wastewater using electrochemical technology: A review
Highlights
► Conventional nitrate and organic carbon removal generate environmental issues. ► Electrochemical technology can remove both contaminants in a green manner. ► Mechanisms of contaminant removal in electrochemical reactors are discussed. ► Mechanisms of contaminant removal in bio-electrochemical reactors are discussed. ► 99% removal of organic matter and nitrate achieved with bio-electrochemical cell.
Introduction
Aquaculture globally has undergone tremendous growth during the last 50 years from a production of less than a million tons in the early 1950s to over 50 million tons during the present time. Aquaculture in Malaysia has developed greatly from small scale family oriented businesses to large scale operations. Exports of aquaculture have recorded positive growth rates as much as RM 1,323,280 (US$ 440,000) to RM 1,769,305 (US$ 587,120) from 2007 to 2008 [1], [2]. The main categories of fish involved have been shellfish, freshwater fish, marine prawn, marine fish and giant freshwater prawn [3]. Fish or fish based products are a cheap source of animal protein for human growth. Due to Malaysia being surrounded by sea and an ocean, it is easy to have access to fish and fish based products [2]. The total consumption of fish has increased from 49 kg/capita/year to 56 kg/capita/year from year 2000 to 2010. Hence, Malaysia has the highest fish consumption rate in the world [1], [2]. Current local production of fisheries, which has increased from 89% in year 2000 to 94.3% in 2010 [1], is unable to achieve the goal of self sufficiency in the future. This is due to an increase in health awareness and rise of population in the country. Thus, in year 2009, through National Aquaculture Development Plan, the Ministry of Agriculture has roughly allocated RM 358 million (US$ 118,796,770) to establish an aquaculture industrial zone and provide the necessary assistance, including infrastructure to cope with the rising demand [3].
The intensive development in the aquaculture industry has caused major environmental impacts. Wastewater discharged from aquaculture contains nitrogenous compounds (ammonia, nitrite and nitrate), phosphorus and dissolved organic carbon, which cause environmental deterioration at high concentrations [4]. Ammonia (NH3) is the product of fish respiration and decomposition of excess organic matter. Chemoautotrophic bacteria (Nitrosomonas and Nitrobacter) tend to oxidize ammonium ions (NH4+) to nitrite (NO2-) and nitrate (NO3-) ions. Nevertheless, these ions are removed by aquatic plants, algae and bacteria since they assimilate them as a source of nitrogen [5]. These nitrogen compounds are nutrients for generating eutrophication which disrupt aquatic ecosystems in a severe manner [6] as shown in Fig. 1. Animal farming, urban and agricultural runoff, industrial wastes, and sewage effluents also increase the concentration of ammonium, nitrate and nitrite ions in aquatic ecosystems [5]. Several studies have been conducted on the toxicity of nitrate on aquatic animals and results indicate that nitrate reacts with hemoglobin causing shortage of oxygen in their body (methaemoglobin) and finally death [5]. When nitrate enters in human intestines, it is also converted into nitrite under anaerobic conditions and this may lead to methaemoglobinaemia in infants [4], [5]. Besides that, formation of nitrosoamines from nitrite can give rise to cancers of the digestive tract since nitrosamines are the most efficacious carcinogens in mammals [7]. Therefore, World Health Organization (WHO) established the limit for nitrate in drinking water to 10 mg NO3-N/L [7], [8].
Total organic carbon (TOC) is defined as any compound containing carbon atoms except CO2 and related substances such as carbonate, bicarbonate and the like [10]. Various natural and man-made activities result in the presence of dissolved organic carbon in aquaculture wastewater. The major compositions of dissolved organic carbon in aquaculture wastewater are humic-like substances, carbohydrates, protein-like substances, low molecular weight aldehydes, fulvic acids, phenols and organic peroxides [11]. Organic carbon is the energy substrate for many microorganisms and its consumption contributes to the problem of inadequate dissolved oxygen in water bodies that become a threat to aquatic life. In addition, treatment costs increase when dissolved organic carbon in wastewater is high [4], [11], [12], [13]. In this article, the removal of conventional TAN, nitrate and organic matter is reviewed in detail. In addition, the review attempts to compare both electrochemical and bio-electrochemical methods used for TAN or nitrate and total organic removal. Finally, an effective method for simultaneous denitrification and TOC removal in synthetic contaminated water and actual aquaculture wastewater is surveyed and recommendations are put forth with some emphasis on a novel bio-electrochemical reactor.
Section snippets
Conventional TAN and nitrate removal methods
Coagulation, filtration, chlorination, UV and ozone treatment are the common methods applied in wastewater treatment but are not considered advanced enough for TAN and nitrate ions removal. So, there are a few techniques available to remove TAN and nitrate ions that are divided into two main categories: physicochemical and biological (Table 1).
Conventional TOC removal methods
Lowering organic carbon concentrations in recirculating aquaculture systems can enhance denitrification since bacteria that consume organic carbon compete directly for space and oxygen with those bacteria that consume ammonia and nitrite [62]. There are a few practical approaches for removal of low levels of TOC (< 10 ppm) in water, which are adsorption and oxidation. Oxidation treatments include ozonation and UV radiation. These processes are briefly reviewed below.
Electrochemical technology
The conventional methods do help with nitrate and organic carbon removal but the disadvantages include sludge production, high energy demand, unstable performance and frequent maintenance requirements [15], [26]. Hence, research on new methods for nitrate and organic carbon removal in aquaculture wastewater is under way. The past few decades has seen the emergence of electrochemical technology for wastewater treatment. The particular advantages of electrochemical treatment include high
Bio-electrochemical technology
Bio-electrochemical systems (BESs) are divided into two major groups which are microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). They have great potential for simultaneous production of energy as well as for wastewater treatment. These systems use microorganisms for catalysis of electrochemical reactions [97], [98]. In MFCs, chemical energy of organic material in wastewater is converted into electrical energy, while in MECs, external electricity is utilized to generate a
Conclusion
The literature reports several physicochemical and biological methods for the removal of TAN, nitrate and TOC. However, they face critical issues such as membrane fouling or generation of toxic by-products that limit their successful application in the field. The electrochemical method could be a good alternative due to its high efficiency, ambient operating conditions, small equipment sizes, minimal sludge generation and rapid start-up. However, the generation of ammonia and nitrite limit its
Acknowledgement
The authors would like to acknowledge the University of Malaya Research Grant RG096/10AET for funding this research.
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