The seafood industry consists primarily of many small processing plants, with a number of larger plants located near industry and population centers. Numerous types of seafood are processed, such as mollusks (oysters, clams, scallops), crustaceans (crabs and lobsters), saltwater fishes and freshwater fishes.
As in most processing industries, seafood-processing operations produce waste-water containing substantial contaminants in soluble, colloidal and particulate forms. The degree of the contamination depends on the particular operation; it may be small (e.g. washing operations), mild (e.g. fish filleting) or heavy (e.g. blood water drained from fish storage tanks).
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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:
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.
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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.
Screening:
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.
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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.
Sedimentation:
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.
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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.
Flow Equalisation:
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.
Separation of Oil and Grease:
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:
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:
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. 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 wastewater during biological operation, this is known as a ‘suspended growth process’, whereas the microorganisms 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 hr/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.
The principles and main characteristics of the most common processes used in seafood-processing waste-water treatment are explained in this article:
Aerobic Process:
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.
Activated Sludge Systems:
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.
Anaerobic Treatment:
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.
Physico-Chemical Treatments:
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 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 floe 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:
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 de-stabilise and coagulate contaminants for easy separation.
Disinfection:
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.
Water is used in the slaughterhouse for carcass washing after hide removal from cattle, calves and sheep and after hair removal from hogs. It is also used to clean the inside of the carcass after evisceration and for cleaning and sanitising equipment and facilities both during and after the killing operation.
Associated facilities such as stockyards, animal pens, the steam plant, refrigeration equipment, compressed air, boiler rooms and vacuum equipment will also produce some waste-water, as will sanitary and service facilities for staff employed on site: these may include toilets, shower rooms, cafeteria kitchens and laboratory facilities. The proportions of water used for each purpose can be variable.
The quantity of waste-water will depend very much on the slaughterhouse design, operational practise and the cleaning methods employed. Waste-water generation rates are usually expressed as a volume per unit of product or per animal slaughtered and there is a reasonable degree of consistency between some of the values reported from reliable sources for different animal types.
Effluents from slaughterhouses and packing houses are usually heavily loaded with solids, floatable matter (fat), blood, manure and a variety of organic compounds originating from proteins. As already stated the composition of effluents depends very much on the type of production and facilities.
The main sources of water contamination are from lairage, slaughtering, hide or hair removal, paunch handling, carcass washing, rendering, trimming and cleanup operations. These contain a variety of readily biodegradable organic compounds, primarily fats and proteins, present in both particulate and dissolved forms.
The waste-water has a high strength, in terms of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Suspended Solids (SS), nitrogen and phosphorus, compared to domestic wastewaters. The actual concentration will depend on in-plant control of water use, by-products recovery, waste separation source and plant management.
In general, blood and intestinal contents arising from the killing floor and the gut room, together with manure from stockyard and holding pens, are separated, as best as possible, from the aqueous stream and treated as solid wastes. This can never be 100 per cent successful, however and these components are the major contributors to the organic load in the wastewater, together with solubilised fat and meat trimmings.
The aqueous pollution load of a slaughterhouse can be expressed in a number of ways. Within the literature reports can be found giving the concentration in waste-water of parameters such as BOD, COD and SS. These, however, are only useful if the corresponding waste-water flow rates are also given.
Even then it is often difficult to relate these to a meaningful figure for general design, as the unit of productivity is often omitted or unclear. These reports do, however, give some indication as to the strength of waste-waters typically encountered and some of their particular characteristics, which can be useful in making a preliminary assessment of the type of treatment process most applicable.
At best it can be concluded that slaughterhouse waste-waters have a pH around neutral, an intermediate strength in terms of COD and BOD, are heavily loaded with solids and are nutrient-rich.
The waste-water contains a high density of total coliform, fecal coliform and fecal streptococcus groups of bacteria due to the presence of manure material and gut contents. Numbers are usually in the range of several million Colony Forming Units (CFU) per 100 ml.
It is also likely that the waste-water will contain bacterial pathogens of enteric origin such as Salmonella sp., and Campylobacter jejuni, gastrointestinal parasites including Ascaris sp., Giardia lamblia and Cryptosporidium parvum and enteric viruses. It is, therefore, essential that slaughterhouse design ensures the complete segregation of process wash water and strict hygiene procedures to prevent cross-contamination.
The mineral chemistry of the waste-water is influenced by the chemical composition of the slaughterhouse’s treated water supply, waste additions such as blood and manure, which can contribute to the heavy metal load in the form of copper, iron, manganese, arsenic and zinc and process plant and pipework, which can contribute to the load of copper, chromium, molybdenum, nickel, titanium and vanadium.
The overall waste load arising from a slaughterhouse is determined principally by the type and number of animals slaughtered. The partitioning of this load between the solid and aqueous phases will depend very much upon the operational practices adopted, however, and there are measures that can be taken to minimise waste-water generation and the aqueous pollution load.
Minimisation can start in the holding pens by reducing the time that the animals remain in these areas through scheduling of delivery times. The incorporation of slatted concrete floors laid to falls of 1 in 60 with drainage to a slurry tank below the floor in the design of the holding pens can also reduce the amount of washdown water required.
Alternatively, it is good practice to remove manure and lairage from the holding pens or stockyard in solid form before washing down. In the slaughterhouse itself, cleaning and carcass washing typically account for over 80 per cent of total water use and effluent volumes in the first processing stages.
One of the major contributors to organic load is blood, which has a COD of about 4,00,000 mg/l and washing down of dispersed blood can be a major cause of high effluent strength. Minimisation can be achieved by having efficient blood collection troughs allowing collection from the carcass over several minutes.
Likewise the trough should be designed to allow separate drainage to a collection tank of the blood and the first flush of wash water. Only residual blood should enter a second drain for collection of the main portion of the wash water.
An efficient blood recovery system could reduce the aqueous pollution load by as much as 40 per cent compared to a plant of similar size that allows the blood to flow to waste. The second area where high organic loads into the waste-water system can arise is in the gut room.
Most cattle and sheep abattoirs clean the paunch (rumen), manyplies (omasum) and reed (abomasum) for tripe production. A common method of preparation is to flush out the gut manure from the punctured organs over a mechanical screen and allow water to transport the gut manure to the effluent treatment system.
Typically the gut manure has a COD of over 1,00,000 mg/l, of which 80 per cent dissolves in the wash water. Significant reductions in waste-water strength can be made by adopting a ‘dry’ system for removing and transporting these gut manures.
The paunch manure in its undiluted state has enough water present to allow pneumatic transport to a ‘dry’ storage area where a compactor can be used to reduce the volume further if required. The tripe material requires washing before further processing, but with a much reduced volume of water and resulting pollution load.
The small and large intestines are usually squeezed and washed for use in casings. To reduce water, washing can be carried out in two stages: a primary wash in a water bath with continuous water filtration and recirculation, followed by a final rinse in clean potable water.
Other measures that can be taken in the gut room to minimise water use and organic loadings to the aqueous stream include ensuring that mechanical equipment, such as the hasher machine, are in good order and maintained regularly.
Other methods can also be employed to minimise water usage. These will not in themselves reduce the organic load entering the waste-water treatment system, but will reduce the volume requiring treatment and possibly influence the choice of treatment system to be employed. For example, high-strength, low-volume waste-waters may be more suited to anaerobic rather than aerobic biological treatment methods.
Water use minimisation methods include:
1. The use of directional spray nozzles in carcass washing, which can reduce water consumption by as much as 20 per cent.
2. Use of steam condensation systems in place of scald tanks for hair and nail removal.
3. Fitting washdown hoses with trigger grips.
4. Appropriate choice of cleaning agents.
5. Reuse of clear water (e.g. chiller water) for the primary washdown of holding pens.
Waste-Water Treatment Processes:
The degree of waste-water treatment required will depend on the proposed type of discharge. Wastewaters received into the sewer system are likely to need less treatment than those having direct discharge into a watercourse.
In the European countries, direct discharges have to comply with the urban waste water treatment directive and other water quality directives. In the United States the EPA has proposed Effluent Limitations Guidelines and Standards (ELGs) for the meat and poultry products industries with direct discharge.
These proposed ELGs will apply to existing and new meat and poultry products (MPP) facilities and are based on the well-tested concepts of ‘best practicable control technology currently available’ (BPT), the ‘best conventional pollutant control technology’ (BCT), the ‘best available technology economically achievable’ (BAT) and the ‘best available demonstrated control technology for new source performance standards’ (NSPS).
In summary, the technologies proposed to meet these requirements use, in the main, a system based on a treatment series comprising flow equalisation, dissolved air flotation and secondary biological treatment for all slaughterhouses; and require nitrification for small installations and additional de-nitrification for complex slaughterhouses.
There is some potential, however, for segregation of waste-waters allowing specific individual pre- treatments to be undertaken or in some cases, bypass of less contaminated streams. Depending on local conditions and regulations, water from boiler houses and refrigerating systems may be segregated and discharged directly or used for outside cleaning operations.
Primary and Secondary Treatment:
Primary Treatment:
Grease removal is a common first stage in slaughterhouse waste-water treatment, with grease traps in some situations being an integral part of the drainage system from the processing areas. Where the option is taken to have a single point of removal, this can be accomplished in one of two ways- by using a baffled tank or by DAF.
A typical grease trap has a minimum detention period of about 30 minutes, but the period need not to be greater than 1 hour. Within the tank, coagulation of fats is brought about by cooling, followed by separation of solid material in baffled chambers through natural flotation of the less dense material, which is then removed by skimming.
Secondary Treatment:
Secondary treatment aims to reduce the BOD of the waste-water by removing the organic matter that remains after primary treatment. This is primarily in a soluble form. Secondary treatment can utilise physical and chemical unit processes, but for the treatment of meat wastes biological treatment is usually favoured.
Physico-Chemical Secondary Treatment:
Chemical treatment of meat-plant wastes is not a common practice due to the high chemical costs involved and difficulties in disposing of the large volumes of sludge produced.
Biological Secondary Treatment:
Using biological treatment, more than 90 per cent efficiency can be achieved in pollutant removal from slaughterhouse wastes. Commonly used systems include lagoons (aerobic and anaerobic), conventional activated sludge, extended aeration, oxidation ditches, sequencing batch reactors and anaerobic digestion.
A series of anaerobic biological processes followed by aerobic biological processes is often useful for sequential reduction of the BOD load in the most economic manner, although either process can be used separately. Slaughterhouse waste-waters vary in strength considerably depending on a number of factors.
For a given type of animal, however, this variation is primarily due to the quantity of water used within the abattoir, as the pollution load (as expressed as BOD) is relatively constant on the basis of live weight slaughtered. Hence, the more economical an abattoir is in its use of water, the stronger the effluent will be and vice versa.
The strength of the organic degradable matter in the wastewater is an important consideration in the choice of treatment system. To remove BOD using an aerobic biological process involves supplying oxygen (usually as a component in air) in proportion to the quantity of BOD that has to be removed, an increasingly expensive process as the BOD increases.
On the other hand an anaerobic process does not require oxygen in order to remove BOD as the biodegradable fraction is fermented and then transformed to gaseous end products in the form of carbon dioxide (CO2) and methane (CH4).
Anaerobic Treatment:
Anaerobic digestion is a popular method for treating meat industry wastes. Anaerobic processes operate in the absence of oxygen and the final products are mixed gases of methane and carbon dioxide and a stabilised sludge.
Anaerobic digestion of organic materials to methane and carbon dioxide is a complicated biological and chemical process that involves three stages: hydrolysis, acetogenesis, and finally methanogenesis. During the first stage, complex compounds are hydrolysed to smaller chain intermediates. In the second stage acetogenic bacteria convert these intermediates to organic acids and then ultimately to methane and carbon dioxide via the methanogenesis phase (Fig. 17.20).
In most of the countries, anaerobic systems using simple lagoons are by far the most common method of treating abattoir waste-water. These are not particularly suitable for use in the heavily populated regions of Western Europe due to the land area required and also because of the difficulties of controlling odours in the urban areas where abattoirs are usually located.
The extensive use of anaerobic lagoons demonstrates the amenability of abattoir waste-waters to anaerobic stabilisation, however, with significant reductions in the BOD at a minimal cost.
The anaerobic lagoon consists of an excavation in the ground, giving a water depth of between 10 and 17 ft (3-5 metres), with a retention time of 5-15 days. Common practice is to provide two ponds in series or parallel and sometimes linking these to a third aerobic pond.
The pond has no mechanical equipment installed and is unmixed except for some natural mixing brought about by internal gas generation and surface agitation; the latter is minimised where possible to prevent odour formation and re-aeration.
Influent waste-water enters near the bottom of the pond and exits near the surface to minimise the chance of short-circuiting. Anaerobic ponds can provide an economic alternative for purification.
The BOD reductions vary widely, although excellent performance has been reported in some cases, with reductions of up to 97 per cent in BOD, up to 95 per cent in SS and up to 96 per cent in COD from the influent values.
Anaerobic lagoons are not without potential problems, relating to both their gaseous and aqueous emissions. As a result of breakdown of the waste-water, methane and carbon dioxide are both produced. These escape to the atmosphere, thus contributing to greenhouse gas emissions, with methane being 25 times more potent than carbon dioxide in this respect.
Gaseous emissions also include the odouriferous gases, hydrogen sulphide and ammonia. The lagoons generally operate with a layer of grease and scum on the top, which restricts the transfer of oxygen through the liquid surface, retains some of the heat and helps prevent the emission of odour.
The use of fabricated anaerobic reactors for abattoir waste-water treatment is also well established. To work efficiently these are designed to operate either at mesophilic (around 95°F or 35°C) or thermophilic (around 130°F or 55°C) temperatures. Anaerobic filters have also been applied to the treatment of slaughterhouse waste-waters.
These maintain a long SRT by providing the micro-organisms with a medium that they can colonise as a biofilm. Unlike conventional aerobic filters, the anaerobic filter is operated with the support medium submerged in an upflow mode of operation.
Because anaerobic filters contain a support medium, there is potential for the interstitial spaces within the medium to become blocked and effective pre-treatment is essential to remove suspended solids as well as solidifiable oils, fats and grease.
The third type of high-rate anaerobic system that can be applied to slaughterhouse waste-waters is the upflow anaerobic sludge blanket reactor (UASB). This is basically an expanded-bed reactor in which the bed comprises anaerobic micro-organisms, including methanogens, which have formed dense granules.
The mechanisms by which these granules form are still poorly understood, but they are intrinsic to the proper operation of the process. The influent waste-water flows upward through a sludge blanket of these granules, which remain within the reactor as their settling velocity is greater than the up flow velocity of the waste-water. The reactor therefore exhibits a long sludge retention time, high biomass density per unit reactor and can operate at a short HRT.
Aerobic Treatment:
Aerobic biological treatment for the treatment of biodegradable wastes has been established for over a hundred years and is accepted as producing a good-quality effluent, reliably reducing influent BOD by 95 per cent or more.
Aerobic processes can roughly be divided into two basic types: those that maintain the biomass in suspension (activated sludge and its variants) and those that retain the biomass on a support medium (biological filters and its variants).
There is no doubt that either basic type is suitable for the treatment of slaughterhouse waste-water and their use is well documented in works such as Broils and Broughton, where aerobic processes are compared with anaerobic ones. In selecting an aerobic process a number of factors need to be taken into account.
These include the land area available, the head of water available, known difficulties associated with certain waste-water types (such as bulking and stable foam formation), energy efficiency and excess biomass production. It is important to realise that the energy costs of conventional aerobic biological treatment can be substantial due to the requirement to supply air to the process.
It is, therefore, usual to only treat to the standard required, as treatment to a higher standard will incur additional cost. For example, in order to convert ammonia to nitrate requires 4.5 moles of oxygen for every mole of ammonia converted. In effect this means that a 1 mg/l concentration of ammonia has an equivalent BOD of 4.5 mg/l. It is, therefore, only usual to aim for the conversion of ammonia to nitrate when this is required.
The most common aerobic biological processes used for the treatment of meat industry wastes are biological filtration, activated sludge plants, waste stabilisation ponds and aerated lagoons.
Activated Sludge:
The activated sludge process has been successfully used for the treatment of waste-waters from the meat industry for many decades. It generally has a lower capital cost than standard-rate percolating filters and occupies substantially less space than lagoon or pond systems.
In the activated sludge process the waste-waters are mixed with a suspension of aerobic micro-organisms (activated sludge) and aerated. After aeration, the mixed liquor passes to a settlement tank where the activated sludge settles and is returned to the plant inlet to treat the incoming waste.
The supernatant liquid in the settlement tank is discharged as plant effluent. Air can be supplied to the plant by a variety of means, including blowing air into the mixed liquor through diffusers; mechanical surface aeration; and floor-mounted sparge pipes.
All the methods are satisfactory provided that they are properly designed to meet the required concentration of dissolved oxygen in the mixed liquor (greater than 0.5 mg/l) and to maintain the sludge in suspension; for nitrification to occur it may be necessary to maintain dissolved oxygen concentrations above 2.0 mg/l.
The activated sludge process can be designed to meet a number of different requirements, including the available land area, the technical expertise of the operator, the availability of sludge disposal routes and capital available for construction.
The first step in the design of an activated sludge system is to select the loading rate, which is usually defined as the mass ratio of substrate inflow to the mass of activated sludge (on a dry weight basis); this is commonly referred to as the food to micro-organism (F:M) ratio and is usually reported as lb BOD/lb MLSS day (kg BOD/kg MLSS day).
For conventional operation the range is 0.2-0.6; the use of higher values tends to produce a dispersed or nonflocculent sludge and lower values require additional oxygen input due to high endogenous respiration rates. Systems with F:M ratios above 0.6 are sometimes referred to as high rate, while those below 0.2 are known as extended aeration systems.
The latter, despite their higher capital and operating costs are commonly chosen for small installations because of their stability, low sludge production and reliable nitrification. Because of the stoichiometric relationship between F:M ratio and mean cell residence time (MCRT), high-rate plants will have an MCRT of less than 4 days and extended aeration plants of greater than 13 days.
Because of the low growth rates of the nitrifying bacteria, which are also influenced markedly by temperature, the oxidation of ammonia to nitrates (nitrification) will only occur at F:M ratios less than 0.1. It is also sometimes useful to consider the nitrogen loading rate, which for effective nitrification should be in the range 0.03-0.08 lb N/lb MLSS-day (kg N/kg MLSS day).
Conventional plants can be used where nitrification is not critical, for example, as a pre-treatment before sewer discharge. One of the main drawbacks of the conventional activated sludge process, however, is its poor buffering capability when dealing with shock loads.
This problem can be overcome by the installation of an equalisation tank upstream of the process, or by using an extended aeration activated sludge system. In the extended aeration process, the aeration basin provides a 24-30 hours (or even longer) retention time with complete mixing of tank contents by mechanical or diffused aeration.
The large volume combined with a high air input results in a stable process that can accept intermittent loadings. A further disadvantage of using a conventional activated sludge process is the generation of a considerable amount of surplus sludge, which usually requires further treatment before disposal.
Some early work suggested the possible recovery of the biomass as a source of protein, but concerns over the possible transmission of exotic animal diseases would make this unacceptable in Europe. The use of extended aeration activated sludge or aerated lagoons minimises biosolids production because of the endogenous nature of the reactions.
The size of the plant and the additional aeration required for sludge stabilisation does, however, lead to increased capital and operating costs. Considering the high concentrations of nitrogen present in slaughterhouse waste-water, ammonia removal is often regarded as essential from a regulatory standpoint for direct discharge and increasingly there is a requirement for nutrient removal.
It is, therefore, not surprising that most modern day designs are of an extended aeration type so as to promote reliable nitrification as well as to minimise sludge production. Efficient designs will also attempt to recover the chemically bound oxygen in nitrate through the process of denitrification, thus reducing treatment costs and lowering nitrate concentrations in the effluent.
Design criteria and loadings for activated sludge treatment have been widely reported and reliable data can be found in a number of reports.
In recent years, a great deal of interest has been shown in the use of sequencing batch reactors (SBRs) for food-processing waste-waters, as these provide a minimum guaranteed retention time and produce a high-quality effluent. A batch process also often fits well with the intermittent discharge of an industrial process working on one or two shifts.
Advantages are an ideal plug flow that maximises reaction rates, ideal quiescent sedimentation and flow equalisation inherent in the design. Decanting can be achieved using floating outlets and adjustable weirs, floating aerators are commonly employed and an anoxic fill overcomes problems of effluent turbidity as well as providing ideal conditions for denitrification reactions.