Waste-water from seafood-processing operations can be very high in biochemical oxygen demand (BOD), fat, oil and grease (FOG) and nitrogen content. Literature data for seafood processing operations showed a BOD production of 1-72.5 kg of BOD per ton of product. White fish filleting processes typically produce 12.5-37.5 kg of BOD for every ton of product. BOD is derived mainly from the butchering process and general cleaning and nitrogen originates predominantly from blood in the waste-water stream.
Seafood-Processing Waste-Water Characterisation:
Seafood-processing waste-water characteristics that raise concern include pollutant parameters, sources of process waste and types of wastes. In general, the waste-water of seafood-processing waste-water can be characterised by its physico-chemical parameters, organics, nitrogen and phosphorus contents. Important pollutant parameters of the waste-water are five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), fats, oil and grease (FOG) and water usage. As in most industrial waste-waters, the contaminants present in seafood-processing waste-waters are an undefined mixture of substances, mostly organic in nature.
Primary Treatment of Wastewater in Seafood Industry:
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In the treatment of seafood-processing waste-water, one should be cognizant of the important constituents in the waste stream. This waste-water contains considerable amounts of insoluble suspended matter, which can be removed from the waste stream by chemical and physical means. For optimum waste removal, primary treatment is recommended prior to a biological treatment process or land application. A major consideration in the design of a treatment system is that the solids should be removed as quickly as possible.
It has been found that the longer the detention time between waste generation and solids removal, the greater the soluble BOD5 and COD with corresponding reduction in by-product recovery. For seafood-processing waste-water, the primary treatment processes are screening, sedimentation, flow equalisation and dissolved air flotation. These unit operations will generally remove up to 85 per cent of the total suspended solids and 65 per cent of the BOD5 and COD per cent in the waste-water.
The removal of relatively large solids (0.7 mm or larger) can be achieved by screening. This is one of the most popular treatment systems used by food-processing plants, because it can reduce the amount of solids being discharged quickly. Usually, the simplest configuration is that of flow-through static screens, which have openings of about 1 mm. Sometimes a scrapping mechanism may be required to minimise the clogging problem in this process.
Fish solids dissolve in water with time; therefore, immediate screening of the waste streams is highly recommended. Likewise, high-intensity agitation of waste streams should be minimised before screening or even settling, because they may cause breakdown of solids rendering them more difficult to separate. In small-scale fish-processing plants, screening is often used with simple settling tanks.
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Sedimentation separates solids from water using gravity settling of the heavier solid particles. In the simplest form of sedimentation, particles that are heavier than water settle to the bottom of a tank or basin. Sedimentation basins are used extensively in the waste-water treatment industry and are commonly found in many flow-through aquatic animal production facilities. This operation is conducted not only as part of the primary treatment, but also in the secondary treatment for separation of solids generated in biological treatments, such as activated sludge or trickling filters.
A flow equalisation step follows the screening and sedimentation processes and precedes the dissolved air flotation (DAF) unit. Flow equalisation is important in reducing hydraulic loading in the waste stream. Equalisation facilities consist of a holding tank and pumping equipment designed to reduce the fluctuations of the waste streams. The equalising tank will store excessive hydraulic flow surges and stabilise the flow rate to a uniform discharge rate over a 24-hour day. The tank is characterised by a varying flow into the tank and a constant flow out.
iv. Separation of Oil and Grease:
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Seafood-processing waste-waters contain variable amounts of oil and grease, which depend on the process used, the species processed and the operational procedure. Gravitational separation may be used to remove oil and grease, provided that the oil particles are large enough to float towards the surface and are not emulsified; otherwise, the emulsion must be first broken by pH adjustment. Heat may also be used for breaking the emulsion but it may not be economical unless there is excess steam available.
Flotation is one of the most effective removal systems for suspensions that contain oil and grease. The most common procedure is that of dissolved air flotation (DAF), which is a waste treatment process in which oil, grease and other suspended matter are removed from a waste stream.
Biological Treatment of Wastewater in Seafood Industry:
To complete the treatment of the seafood-processing waste-waters, the waste stream must be further processed by biological treatment. Biological treatment involves the use of micro-organisms to remove dissolved nutrients from a discharge. Organic and nitrogenous compounds in the discharge can serve as nutrients for rapid microbial growth under aerobic, anaerobic or facultative conditions.
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The three conditions differ in the way they use oxygen. Aerobic micro-organisms require oxygen for their metabolism, whereas anaerobic micro-organisms grow in absence of oxygen; the facultative micro-organism can proliferate either in absence or presence of oxygen although using different metabolic processes.
The biological treatment processes used for waste-water treatment are broadly classified as aerobic and anaerobic treatments. Aerobic and facultative micro-organisms predominate in aerobic treatments, while only anaerobic micro-organisms are used for the anaerobic treatments.
If micro-organisms are suspended in the waste-water during biological operation, this is known as a ‘suspended growth process’, whereas the micro-organisms that are attached to a surface over which they grow are said to undergo an ‘attached growth process’.
Biological treatment systems are most effective when operating continuously 24 hours/day and 365 days/year. Systems that are not operated continuously have reduced efficiency because of changes in nutrient loads to the microbial biomass. Biological treatment systems also generate a consolidated waste stream consisting of excess microbial biomass, which must be properly disposed. Operation and maintenance costs vary with the process used.
In seafood processing waste-waters, the need for adding nutrients (the most common being nitrogen and phosphorus) seldom occurs, but an adequate provision of oxygen is essential for successful operation. The most common aerobic processes are activated sludge systems, lagoons, trickling filters and rotating disc contactors.
In an activated sludge treatment system, an acclimatised, mixed, biological growth of micro-organisms (sludge) interacts with organic materials in the waste-water in the presence of excess dissolved oxygen and nutrients (nitrogen and phosphorus). The micro-organisms convert the soluble organic compounds to carbon dioxide and cellular materials. Oxygen is obtained from applied air, which also maintains adequate mixing. The effluent is settled to separate biological solids and a portion of the sludge is recycled; the excess is wasted for further treatment such as dewatering.
Aerated lagoons are used where sufficient land is not available for seasonal retention or land application and economics do not justify an activated sludge system. Efficient biological treatment can be achieved by the use of the aerated lagoon system.
c. Stabilisation/Polishing Ponds:
A stabilisation/polishing ponds system is commonly used to improve the effluent treated in the aerated lagoon. This system depends on the action of aerobic bacteria on the soluble organics contained in the waste stream. The organic carbon is converted to carbon dioxide and bacterial cells. Algal growth is stimulated by incident sunlight that penetrates to a depth of 1-1.5 metre. Photosynthesis produces excess oxygen, which is available for aerobic bacteria; additional oxygen is provided by mass transfer at the air-water interface.
The trickling filter is one of the most common attached cell (biofilm) processes. Unlike the activated sludge and aerated lagoons processes, which have biomass in suspension, most of the biomass in trickling filters are attached to certain support media over which they grow.
e. Rotating Biological Contactors (RBC):
Increasingly stringent requirements for the removal of organic and inorganic substances from wastewater have necessitated the development of innovative, cost-effective waste-water treatment alternatives in recent years. The aerobic rotating biological contactor is one of the biological processes for the treatment of organic waste-water. It is another type of attached growth process that combines advantages of biological fixed-film (short hydraulic retention time, high biomass concentration, low energy cost, easy operation and insensitivity to toxic substance shock loads) and partial stir. Therefore, the aerobic RBC reactor is widely employed to treat both domestic and industrial waste-water.
Anaerobic biological treatment has been applied to high BOD or COD waste solutions in a variety of ways. Treatment proceeds with degradation of the organic matter, in suspension or in a solution of continuous flow of gaseous products, mainly methane and carbon dioxide, which constitute most of the reaction products and biomass. Its efficient performance makes it a valuable mechanism for achieving compliance with regulations for contamination of recreational and seafood-producing wastes.
Anaerobic treatment is the result of several reactions – the organic load present in the waste-water is first converted to soluble organic material, which in turn is consumed by acid-producing bacteria to produce volatile fatty acids, plus carbon dioxide and hydrogen. The methane-producing bacteria consume these products to produce methane and carbon dioxide. Typical micro-organisms used in this methanogenic process are Metanobacterium, Methanobacillus, Metanococcus, and Methanosarcina.
Anaerobic digestion facilities have been used for the management of animal slurries for many years, they can treat most easily biodegradable waste products, including everything of organic or vegetable origin. Recent developments in anaerobic digestion technology have allowed the expansion of feed-stocks to include municipal solid wastes, bio-solids and organic industrial waste (e.g. seafood-processing wastes).
Physico-Chemical Treatment of Wastewater in Seafood Industry:
a. Coagulation/Flocculation:
Coagulation or flocculation tanks are used to improve the treatability of waste-water and to remove grease and scum from waste-water. In coagulation operations, a chemical substance is added to an organic colloidal suspension to destabilise it by reducing forces that keep them apart, that is, to reduce the surface charges responsible for particle repulsions. This reduction in charges is essential for flocculation, which has the purpose of clustering fine matter to facilitate its removal. Particles of larger size are then settled and clarified effluent is obtained.
In seafood processing waste-waters, the colloids present are of an organic nature and are stabilised by layers of ions that result in particles with the same surface charge, thereby increasing their mutual repulsion and stabilisation of the colloidal suspension. This kind of waste-water may contain appreciable amounts of proteins and micro-organisms, which become charged due to the ionisation of carboxyl and amino groups or their constituent amino acids. The oil and grease particles, normally neutral in charge, become charged due to preferential absorption of anions, which are mainly hydroxyl ions.
Several steps are involved in the coagulation process. First, coagulant is added to the effluent and mixing proceeds rapidly and with high intensity. The purpose is to obtain intimate mixing of the coagulant with the waste-water, thereby increasing the effectiveness of destabilisation of particles and initiating coagulation.
A second stage follows in which flocculation occurs for a period of up to 30 minutes. In the latter case, the suspension is stirred slowly to increase the possibility of contact between coagulating particles and to facilitate the development of large floes. These floes are then transferred to a clarification basin in which they settle and are removed from the bottom while the clarified effluent overflows.
Several substances may be used as coagulants. The pH of several waste-waters of the proteinaceous nature can be adjusted by adding acid or alkali. The addition of acid is more common, resulting in coagulation of the proteins by denaturing them, changing their structural conformation due to the change in their surface charge distribution. Thermal denaturation of proteins can also be used, but due to its high energy demand, it is only advisable if excess steam is available. In fact, the ‘cooking’ of the blood- water in fishmeal plants is basically a thermal coagulation process.
Another commonly used coagulant is polyelectrolyte, which may be further categorised as cationic and anionic coagulants. Cationic polyelectrolytes act as a coagulant by lowering the charge of the waste-water particles, because waste-water particles are negatively charged. Anionic or neutral polyelectrolyte are used as bridges between the already formed particles that interact during the flocculation process, resulting in an increase of floc size.
Since the recovered sludges from coagulation/flocculation processes may sometimes be added to animal feeds, it is advisable to ensure that the coagulant or flocculant used is not toxic.
Electrocoagulation (EC) has also been investigated as a possible means to reduce soluble BOD. It has been demonstrated to reduce organic levels in various food- and fish-processing waste streams. During testing, an electric charge was passed through a spent solution in order to destabilise and coagulate contaminants for easy separation.
Disinfection of seafood-processing waste-water is a process by which disease-causing organisms are destroyed or rendered inactive. Most disinfection systems work in one of the following four ways – (i) damage to the cell wall, (ii) alteration of cell permeability, (iii) alteration of the colloidal nature of protoplasm, and (iv) inhibition of enzyme activity.
Disinfection is often accomplished using bactericidal agents. The most common agents are chlorine, ozone (O3) and ultraviolet (UV) radiation.
Chlorination is a process commonly used in both industrial and domestic waste-waters for various reasons. In fisheries effluents, however, its primary purpose is to destroy bacteria or algae or to inhibit their growth. Usually the effluents are chlorinated just before their final discharge to the receiving water bodies. For this process either chlorine gas or hypochlorite solutions may be used, the latter being easier to handle. In waste solutions, chlorine forms hypochlorous acid, which in turn forms hypochlorite.
Ozone (O3) is a strong oxidising agent that has been used for disinfection due to its bactericidal properties and its potential for removal of viruses. It is produced by discharging air or oxygen across a narrow gap with application of a high voltage. An ozonation system is presented in Fig. 28.1.
Ozonation has been used to treat a variety of waste-water streams and appears to be most effective when treating more dilute types of wastes. It is a desirable application as a polishing step for some seafood-processing waste-waters, such as from liquid-processing operations, which is fairly concentrated.
Ozone reverts to oxygen when it has been added and reacted, thus increasing somewhat the dissolved oxygen level of the effluent to be discharged, which is beneficial to the receiving water stream. Contact tanks are usually closed to recirculate the oxygen-enriched air to the ozonation unit. Advantages of ozonation over chlorination are that it does not produce dissolved solids and is affected neither by ammonia compounds present nor by the pH value of the effluent. On the other hand, ozonation has been used to oxidise ammonia and nitrites presented in fish culture facilities.
Ozonation also has limitations. Because ozone’s volatility does not allow it to be transported, this system requires ozone to be generated onsite, which requires expensive equipment. Although much less used than chlorination in fisheries waste-waters, ozonation systems have been installed in particular in discharges to sensitive water bodies.
f. Ultraviolet (UV) Radiation:
Disinfection can also be accomplished by using ultraviolet (UV) radiation as a disinfection agent. UV radiation disinfects by penetrating the cell wall of pathogens with UV light and completely destroying the cell and/or rendering it unable to reproduce.
Economic Considerations of Seafood-Processing and Meat Processing Waste-Water Treatment:
Economic considerations are always the most important parameters that influence the final decision as to which process should be chosen for waste-water treatment. In order to estimate cost, data from the waste-water characterisation should be available together with the design parameters for alternative processes and the associated costs. Costs related to these alternative processes and information on the quality of effluent should also be obtained prior to cost estimation in compliance with local regulations.
During the design phase of a waste-water treatment plant, different process alternatives and operating strategies could be evaluated by several methods. This cost evaluation can be achieved by calculating a cost index using commercially available software packages. Nevertheless, actual cost indices are often restrictive, since only investment or specific operating costs are considered. Moreover, time-varying waste-water characteristics are not directly taken into account but rather through the application of large safety factors.
Finally, the implementation of adequate control strategies such as a real-time control is rarely investigated despite the potential benefits. In order to avoid these problems, a concept of model-based simulation system for cost calculation (MoSS-CC) was introduced by Gillot, which is a modelling and simulation tool aimed at integrating the calculation of investment and fixed and variable operating costs of a waste-water treatment plant. This tool helps produce a holistic economic evaluation of a waste-water treatment plant over its life cycles.