Characteristics of Agricultural and Food Wastewater

Whenever and wherever food, in any form, is handled, processed, packed and stored, there will always be an unavoidable generation of wastewater. Wastewater is the most serious environmental problem in the manufacturing and processing of foods. Most of the volume of wastewater comes from cleaning operations at almost every stage of food processing and transportation operations. The quantity and general quality (i.e., pollutant strength, nature of constituents) of this processing wastewater generated have both economic and environmental consequences with respect to its treatability and disposal.

The cost for treating the wastewater depends on specific characteristics of it. Two significant characteristics that dictate the cost for treatment are the daily volume of discharge and the relative strength of the wastewater. Other characteristics become important as system operations are affected and specific discharge limits are identified (i.e., suspended solids). The environmental consequences in inadequately removing the pollutants from the waste stream can have serious ecological ramifications. For example, if inadequately treated wastewater were to be discharged to a stream or river, a eutrophic condition would develop within the aquatic environment due to the discharge of biodegradable, oxygen-consuming materials. If this condition were sustained for an extended period of time, the ecological balance of the receiving stream, river, or lake (i.e., aquatic microflora, plants, and animals) would be upset. Continual depletion of the oxygen in these waters would also give rise to the development of obnoxious odors and unsightly scenes.

Knowledge of characteristics of food and agricultural wastewater is essential to the development of economical and technically viable waste-water management systems that are in compliance with current environmental policy and regulations. Management methods that may have been adequate with other industrial wastewaters may be less feasible with food and agricultural wastewater unless the methods are modified to reflect the characteristics of the wastewater and opportunities it may hold. The waste-waters produced in agricultural processing and food processing vary in quantity and quality, with those streams from food processing typically having low strength and high volume and those coming from animal farming operations tending to have high strength and low volume. These differences in quantity and quality dictate the type and capacity of waste-water management systems that should be deployed.

A clear understanding of the characteristics of food and agricultural wastewater permits management decision on treatment and utilization methods that are effective and economical; this point is further spelled out in Table 1.1. For example, a low-strength, high-volume wastewater containing a small amount of organic colloidal particulates may require a stand-alone biological wastewater treatment facility or just a plate-and-frame filter press; the decision is both technical and economical. Another generalized observation is that the bulk of oxygen-demanding substances is in the liquid phase for food processing wastewater; most oxygen-demanding substances in the wastewater of a high-intensity livestock farming operation are in the form of solid particulates. Some food processing operations occur seasonally (processing of fruits and vegetables); this seasonality adds complexity to the wastewater management systems that handle different sources of food and agricultural wastewater year-round, and clearly the understanding of wastewater characteristics helps plan ahead for this abnormality of process operations. Knowledge of wastewater characteristics also allows strategic planning of water recycling and reuse and recovery of valuable components in the wastewater.

As in most wastewaters, the components present in agricultural and food wastewater run a gamut of many undefined substances, almost all organic in nature. Organic matters are substances containing compounds in possession of mainly elements C, H, and O. The carbon atoms in the organic matters (also called carbonaceous compounds) may be oxidized both chemically and biologically to yield CO2 and energy. It is possible that some sources of wastewaters from certain food processing operations in a processing plant may have limited numbers of possible contaminants present; however, these wastewaters tend to mix with other streams of wastewaters

Table 1.1. Wastewater treatment options available to remove various categories of pollutants in food and agricultural wastewater.

Pollutants in Wastewaters

Management Options

Dissolved organic species Dissolved inorganic species

Suspended organic materials

Suspended inorganic materials

Biological treatment; adsorption; land applications; recovery and utilization Ion exchange; reverse osmosis; evaporation/

distillation; adsorption Physicochemical treatment; biological treatment; land applications; recovery and utilization Pretreatment (screen); physicochemical treatment (sedimentation, flotation, filtration, coagulation)

from the same work site, making it virtually impossible to catalog the substances in the effluents from the plant. Thus, the characteristics of agricultural and food wastewater can be viewed as a set of well-defined physico-chemical and biological parameters that are critical in designing and managing agricultural and food wastewater treatment facilities.

General characteristics of wastewaters in agriculture and food processing

Wastewater from food processing operations is defined by the food itself. Food and agricultural wastewater contains dissolved organic solids from various operations and debris from mechanical processing of foods, such as peeling and trimming, and hydrodynamic impacts in washing and transporting. Agricultural and food processing operations inevitably use large quantities of water to wash—and, in some instances, cool—food items. Canning wastewaters are essentially the same as home kitchen waste because the wastewater is accumulated from various processes involved in the canning operations, such as trimming, sizing, juicing, pureeing, blanching, and cooking. Vegetables also require large amounts of water to blanch and cool. Almost all operations in food or agricultural processing involve cleaning plant floors, machinery, and processing areas; the water used is often mixed with detergents that sometimes are doubled as lubricants for the food processing machinery.

Depending on particular processing operations, water used in the operations is often reused with or without treatment when this practice is economical and legal. As fresh water supply diminishes in many parts of the world, water reuse is often seen as a must for all practical reasons. Reusing and recycling water can result in a considerable reduction in water usage; however, one should keep in mind that if the reused water is intended for edible food items, the food safety issue arising from the reused water should be examined diligently and thoroughly. After all, food safety remains the overriding concern in all food processing and manufacturing operations.

There are common pollutants present in the majority of food and agricultural wastewater and effluents from each stage of the typical waste-water treatment processes (see the following chapters for more information); they are free and emulsified oil/grease, suspended solids, organic colloids, dissolved inorganics, acidity or alkalinity, and sludges. Table 1.1 is a summary of the processes available to treat food and agricultural wastewater.

Each food processing plant produces wastewaters of different quantity and quality. No two plants, even with similar processing capacity of food products, will generate wastewaters of the same quantity and quality because there are too many variables (technical or otherwise) in the process that ultimately define characteristics of wastewater. Furthermore, even different periods of food processing in the same plant may produce different wastewater streams with different characteristics. It is therefore essential to understand that the generalized description of wastewaters from fruit and vegetable processing needs to be understood as an approximation to explaining a complex issue. Any quantitative information shown here or anywhere else shall be considered as averaged data. Typical characteristics, estimated volume, and estimated organic loading of wastewater generated by the food processing industry in the state of Georgia, U.S.A., are tabulated in Table 1.2.

All major food and agricultural processing operations generate waste-water streams; however, the amount and strength of the wastewater streams vary with the major segments of the food and agricultural processing industry. Table 1.3 summarizes the sources of the wastewater streams and possible treatment processes. As shown in Table 1.3, not all agro-food processing operations generate substantial wastewater that warrants on-site wastewater treatment facilities. The following summary of the major segments of the agro-food processing operations requiring

Table 1.2. Typical characteristics, estimated volume, and estimated organic loading of wastewater generated by the food processing industry in Georgia (source: Magbunua, 2000).

Estimated

Estimated

Wastewater

Organic

Volume,

Loading,

Million

Typical

Tons/Year

Industry Group

Gallons/year

Characteristics

BOD

Meat and poultry products

10,730

1,800 mg/l BOD 1,600 mg/l TSS 1,600 mg/l FOG 60 mg/l TKN

80,600

Dairy products

500

2,300 g/l BOD 1,500 mg/l TSS 700 mg./l FOG

14,900

Canned, frozen and preserved

2,080

500 mg/l BOD

4,300

fruits and vegetables

1,100 mg/l TSS

Grain and grain mill products

130

700 mg/l BOD 1,000 mg/l TSS

300

Bakery products

530

2,000 mg/l BOD 4,000 mg/l TSS

4,400

Sugar and confectionery products

140

500 mg/l BOD

300

Fats and oils

350

4,100 g/l BOD 500 mg./l FOG

7,000

Beverages

3,660

8,500 mg/l BOD

91,000

Miscellaneous food preparations

700

6,000 mg/l BOD

5,600

and kindred products

3,000 mg/l TSS

TOTAL

18,810

208,600

Abbreviations: BOD: biological oxygen demand; TSS: total suspended solids; FOG: fats, oils, and grease, TKN: total Kjeldahl nitrogen.

wastewater treatment is presented for the reader to appreciate the unique pollution issues in these segments, even though it is clear that there is considerable similarity among many segments of the food and agricultural processing industry. Additional information about the characteristics of wastewaters in all major segments of the food and agricultural processing industry can be found from Middlebrooks' book (1979).

Table 1.3. Summary of wastewater sources and treatment strategies in major food and agricultural processing.

Agro-Food Sources of

Operations Wastewater Streams Treatment Strategies

Vegetables and fruits

Fishery

Poultry and meat

Dairy

Corn wet milling

Sorting, trimming, washing, peeling, pureeing, in-plant transport, canning and retort, dehydration, and cleanup Eviscerating, trimming, washing, precooking, canning and retort, and cleanup Animal waste, killing and bleeding, scalding (poultry), eviscerating, washing, chilling, and cleanup By-products, spills, leaks, line cleaning, and cleanup Steeping water, washing, and cleanup

Sugar refining Process water and cooling water

Oil and fat

Nonalcoholic beverage Alcohol beverage

Flavoring extracts

Egg product

Other food production

Steaming, solvent recovery, degumming, soapstock water, neutralization, and cleanup

Cleanup

Washing, cooling, leaks, and cleanup

Washing, evaporator condensate, steam distillation, and cleanup

Washing, leaks, and cleanup

Leaks and cleanup

Primary and secondary treatment processes

Primary and secondary treatment processes

Primary and secondary treatment processes

Biological wastewater treatment Mainly screen, activated sludge processes, and secondary sedimentation Recycling and discharge to municipal waste-water systems Primary, secondary treatment, and sludge treatment processes Discharge to municipal wastewater systems Biological wastewater treatment and stabilization ponds Biological wastewater treatment or direct discharge to municipal wastewater systems Biological wastewater treatment and aerobic lagoon Depending on specific products and locality

Table 1.4. Common unitary processes of fruit and vegetable processing that generate wastewater.

Process

Wastewater Comes From . . .

Washing and rinsing

The entire process; may use detergent or chlorinated

water

Sorting (grading)

Density grading operation only

In-plant transport

Water conveys products from one location to the

other

Peeling

Hot water or high-pressure water spray; may involve

chemicals (caustic soda) or detergents

Pureeing and juicing

Condensated evaporated water

Blanching

Hot water or steam for blanching

Canning and retort

Washing cans and steam for retort and cooling with

water

Drying or dehydration

Condensated evaporated water

Mixing and cooking

Leaking of liquid products

Cleanup

Cleaning up at every stage

Wastewaters from fruit and vegetable processing

The fruit and vegetable industries are as assorted as the names imply— these industries process in a number of ways the great variety of fruits and vegetables grown in the United States. The categories of processing include canning, freezing, dehydrating, and pickling and brining. The quantity and quality of wastewater streams from the industries vary considerably with the operations of the processing and seasons.

Fruit and vegetable processing plants are major water users and waste generators. In all stages of food processing (unitary processes), raw foods must be rendered clean and wholesome and food processing plants must be maintained sanitary all the time. Several common unitary operations of fruit and vegetable processing that generate wastewater are shown in Fig. 1.1.

Some of these unitary operations are intuitively obvious waste generating (e.g., washing and rinsing); others are less so (e.g., in-plant transport). Table 1.4 provides a brief explanation of several unitary processes that generate wastewater. For the most part, these wastewaters have been shown to be biodegradable, although salt is not generally removed during the treatment of olive storage and processing brines, cherry brines, and sauerkraut brines.

Figure 1.1. Unitary processes of fruit and vegetable processing that generate wastewater.

The effluents from fruit and vegetable processing operations consist of mainly carbohydrates, such as sugars, starches, pectins, vitamins, and other components of the cell walls that have been severed during processing. Of the total organic matters, 70-80% is in the dissolved form; they are not easily removed from wastewater by conventional mechanical means, although physicochemical processes such as adsorption and chemical oxidation or membrane-based technologies such as membrane filtration are capable of removing dissolved solids in relatively low concentrations at higher costs (see Chapter 3 for adsorption and chemical oxidation and membrane filtration). Obviously, biological wastewater treatment methods will work best in this type of wastewater streams.

The majority of the literature review regarding characterization of fruit and vegetable processing wastewaters focuses on wastewater streams from canning of fruits and vegetables (e.g., Soderquist et al., 1975); the wastewaters from other processing operations of fruits and vegetables are of importance as well. Blanching of vegetables for freezing is a process that requires a large amount of water, and the quantity of wastewater generated is also proportionally high. Fig. 1.2 shows a flow diagram of water reuse

Figure 1.2. A diagram of a four-stage counterflow system for reuse of water in a pea cannery.

in a pea cannery. Postharvesting agricultural wastewater could also be a source of wastewater. Washing and rinsing water used in cleaning fresh produce and fruits are sometimes reused; however, wastewater is still generated in the process and has to be treated eventually. There is a possibility of recovering valuable substance from wastewater streams in fruit and vegetable processing, such as flavors from blanching waters; however, it is often technically complex and economically impractical to extract the valuables among a large number of undesirables in these streams with current technologies.

Wastewaters from the fishery industry

The production processes used in the fishery industry generally include the following: harvesting, storing, receiving, eviscerating or butchering, precooking, picking or cleaning, preserving, and packaging. Harvesting provides the basic raw materials (fishes) for processing and subsequent distribution to the consumer. Once fishes are aboard the fishing vessel, the catch either is taken directly to the processor, or is iced or frozen for later delivery. Preprocessing may be undertaken aboard before the catch is sent to the processing plant. It may include beheading shrimp at sea, eviscerating fish or shellfish at sea, and other operations to prepare the fish for butchering. Wastes from the butchering and evisceration that are sizable are usually collected in the dry form, or screened from the wastewater stream, and processed as a fishery by-product.

The receiving operation usually involves unloading the vessel, weighing, and transporting by conveyor or suitable container to the processing area. The catch may be processed immediately or transferred to cold storage.

Sometimes, cooking or precooking crab and other shellfishes or tuna may be practiced in order to prepare the fish or shellfish for removal of meat and the cleaning operation. The inedible fish or seafood parts, such as skin, bone, gills, shell, and similar parts, are easily removed after pre-cooking. The steam condensate, or stick water, from the tuna or crab pre-cooking is often collected and further processed as a by-product. Wastes generated during this procedure are sometimes collected and saved for byproduct processing. Depending on the species, the cleaning operation may be either manual or mechanical. With fresh fish or fresh shellfish, the meat product is packed into a plastic container and refrigerated for shipment to a distribution center or directly to a retail outlet. If shelf life of the product is required for an extended period of time before consumption, preservation techniques must be used to prevent spoilage from bacterial activities and enzymatic autolysis. Freezing, canning, pasteurization, drying, and refrigeration are the most common preservation techniques used in the fishery industry.

Characteristics of fishery wastewaters are often dependent on several factors: method of processing (mechanical or manual), fish species, and fish products. However, even with similar processing plants using the same method of processing on the same species of fish and producing the same fish products, the quality of wastewaters (in terms of BOD [biological oxygen demand], COD [chemical oxygen demand], or TSS [total suspended solids]) varies with location and even with season. It should be mentioned that there is no substitution for direct determination of the quality of fishery wastewater in the effluent being investigated.

Like all wastewaters under consideration for treatment, the issue of treatability of seafood or fishery wastewater is often shaped by the discharge limits set up by the government agencies or an international body enforced through international treaties. Specifically, the discharge limits of BOD5, TSS, and fat/oil/grease (FOG) are enforced based on the variety of fish species. Table 1.5 is a summary of discharge limits imposed by the United States Environmental Protection Agency (USEPA) in 1985. It is

Table 1.5. Summary of discharge limits for the fishery industry imposed by the United States Environmental Protection Agency in 1985 (source: USEPA).

Fish Species

BOD5 (mg/l)

TSS (%)

FOG (mg/l)

Tuna

20.0

8.3

2.1

Salmon

2.7

2.6

0.31

Other finfish

1.2

3.1-3.6

1.0-43

Crab

0.3-10

2.2-19

0.6-1.8

Shrimp

63-155

110-320

36-126

Clam and oyster

N/A

24-59

0.6-2.4

prudent to consult with the local authorities on issues related to discharge limits of fishery wastewaters.

Fishery wastewaters are rich in fats and proteins. According to Middle-brooks (1979), a processing plant for finfish processing can produce 3.32 kg/ton BOD, 0.348 kg/ton grease/oil, and 1.42 kg/ton suspended solids in the wastewater if using manual processing, or 11.9 kg/ton BOD, 2.48 kg/ton grease/oil, and 8.92 kg/ton suspended solids in the wastewater if using mechanical means. This has generated a lot of interest in recovering these materials to offset totally or partially the costs of treating the fishery wastewater. Like proteins, the presence of FOG in the fishery wastewaters is mainly due to the processing of fishes. Canning, for example, generates grease and oil after fish products are heated.

Wastewaters from meat and poultry processing

The meat and poultry processing industry (excluding rendering but including seafood processing) uses an estimated 150 billion gallons of water annually. Although a portion of the water used by the industry is reused or recycled, most of the water becomes wastewater. Similar to those waste-water streams from the fishery industry, the wastewaters from meat and poultry processing are high in fat/oil/grease and proteins.

The poultry industry handles billions of pounds of chickens (called broilers, weighing from 2.5 lb to 4.5 lb) and turkeys each year, and processing plants vary ranging from 50,000 birds to 250,000 birds per day. The main poultry operations involve receiving and storing, slaughtering, defeathering, eviscerating, packing, and freezing. Nearly all these operations involve using water, and many pollutants in the wastewater stream are created in the receiving and storing operation where manure and un-

Biological Oxygen Demand Test Flowcharts
Figure 1.3. A flow chart of a poultry processing plant.

consumed feed are washed down from the broilers. The water usage and wastewater generation is illustrated in Fig. 1.3.

A meat processing plant consists of a slaughterhouse and/or a packinghouse. The slaughtering process has four basic operations: killing, hide removal/hog dehairing, eviscerating/trimming, and cooling of carcasses (USEPA, 1974). Each of these operations contributes to the wastewater stream; however, before herding the animals to their final destinations, the animals are held in the livestock holding pens generating additional waste-water streams. The wastewater streams from these holding pens primarily come from the spilling from the water troughs, from cleanup, and from wastes laid by the animals.

Wastewaters from the dairy industry

The dairy industry is one of the most important agricultural processing industries in the United States and has grown steadily in the last decades. Wastewaters originate from two major dairy processing—from fluid milk at the receiving station and bottling plants, but increasingly more importantly at the processing plants that produce condensed milk, powdered milk, condensed whey and dry whey, butter, cheese, cultured product, ice cream, and cottage cheese. The milk itself has a BOD5 of 100,000 mg/l and washing plants that produce butter and cheese may produce a waste-water with BOD5 of 1,500 to 3,000 mg/l. Dairy processing uses raw materials beyond milk and milk-related materials; nondairy ingredients such as flavors, sugar, fruits, nuts, and condiments are utilized in manufacturing ice cream, yogurt, and flavored milk, and frozen desserts. The pollutants can enter the wastewater streams through spills, leaks, and wasting of by-products. Apart from whey, which is acidic, most dairy wastewater streams are neutral or slightly alkaline, but they tend to become acidic rapidly due to the lactic acid produced as a result of fermentation of lactose.

Most dairy product processing operations are multiproduct facilities. Among these dairy processing operations there may be receiving stations, bottling plants, creameries, ice cream plants, and cheese-making plants; all these operations may contribute to wastewater streams in the dairy industry. Controlling product loss and recovery of by-products (e.g., whey protein) can improve not only yields, thus profits, but also the amount and strength of dairy wastewater streams.

Dairy wastewaters are amenable to biological wastewater treatment and this is the principal method used in the dairy industry; according to USEPA (1974), there were 64 activated sludge plants, 34 trickling filters, 6 aerobic lagoons, one stabilization pond, 4 combined systems, 2 anaerobic digestion facilities, and 1 sand filtration for secondary effluent operating in the dairy industry in the United States. Most dairy processing plants treat their wastewaters to a level that is acceptable to municipal wastewater treatment facilities.

Wastewaters from oil and fat processing

Edible oil extraction involves solvent extraction of oil-bearing seeds or animal fats (there are mechanical expressers for olive oil and sesame oil) and refining steps of removing undesirables from extracted oil. In addition to cleanup and washing, which use water thus generating wastewater, the following all contribute to wastewater streams in edible oil production plants: deodorization that involves the injection of steam; refining that involves removing free fatty acids, phosphatides, and other impurities with caustic soda; and oil recovery from the extracted meal using water. The waste-waters from the oil production and refining industry, without doubt, are amenable to biological wastewater treatment. There are several pollutants in the wastewater streams from the edible oil extraction and refining; they are free and emulsified oils, grease, suspended solids, and dissolved organic and inorganic solids. Along with the sludges that come from either primary or secondary treatment processes, many common wastewater treatment processes may be employed to remove these pollutants. Trace amounts of solvents such as hexane may be removed by adsorption or steam/air stripping. Another environmentally friendly method of removing hexane from wastewater is pervaporation (Peng et al., 2003).

Parameters for physicochemical treatment of wastewater pH

pH is a measurement of the acidity of the wastewater and an indication of growth conditions for the microbial communities used in biological waste-water treatment regimens. pH values vary greatly with the sources of agricultural and food wastewater and with the environmental conditions and duration of storage of the wastewater collected, because these factors dictate the amount of certain substances and decomposition of biological matters as well as emissions of ammonia compounds.

Solids content

Solids in wastewaters come in two forms: suspended solids (nondissolv-able) and dissolved solids. Suspended solids are nuisances because they can either settle on the bottom of the receiving water body or float on the surface of the water body. Either way will affect the ecological balances of the receiving water body. Solids that readily settle are usually measured with an Imhoff cone, in which a known amount of water sample is poured in the cone and the amount of the solids settled at given times is recorded and compared with the admissible amount of settling solids in the waste-water for discharge. The acceptable settling solids level is usually determined by environmental regulations and as a rule of thumb, discharge of wastewater or treated wastewater is not acceptable if the result of Imhoff testing shows that the water sample contains settling solids after 10 minutes of testing.

Suspended solids are usually measured with a porous fiberglass filter of known pore size, in which a known amount of well-mixed water sample passes through. The dry mass accumulation on the filter is the amount of nondissolvable solids.

Oils and greases represent another realm of suspended solids. These floating substances from some food operations have a tendency to clog the pipes and stick to the surfaces of any material. They are also easily oxidized, producing objectionable odors. In any case, oils and greases should be removed. The amount of oil and grease may be measured with the solvent extraction method found in the standard methods (Eaton et al., 2005).

Soluble solids are laboratory measured with evaporation and subsequent weighing of the remaining dry mass of a known amount of water filtrate sample that is collected from the suspended solids measurement or similar pretreatment to remove suspended solids. Soluble solids are significant in some sources of food wastewater (e.g., fishery, dairy industries) and thus are important in formulating wastewater treatment and resource recovery strategy.

Temperature

It is generally accepted that the temperature of discharged wastewater to a receiving water body cannot exceed 2-3°C of the ambient temperature in order to maintain population balance of the aquatic ecosystem of the receiving water body. Wastewater from some food operations such as retort should be cooled before discharge or biological treatment.

Odor

Odor by itself is not a pollutant, although prolonged and intense exposure has been attributed to adverse effects on wastewater treatment plant workers, and even residents living near the plant, with symptoms such as headache and nausea. Food wastewater contains significant amounts of organic matters and when the organic matters decompose into volatile amines, diamines, and sometimes ammonia or hydrogen sulfide, odor transpires and it can be overwhelming. The other source of wastewater odor generated in food processing is the blanching operation of certain vegetables such as cauliflower and cabbage. The incentive of developing odor abatement strategy in food and agricultural wastewater management is obvious: the public perception and acceptance of a food processing plant is often nostrilic not nostalgic.

Parameters for biological treatment of wastewater

The organic matters in food and agricultural wastewater are considerable and complex. Instead of attempting to identify each organic component of wastewater, wastewater professionals use the parameters for biological wastewater treatment to classify the organic matters. The most common parameters are the oxygen demand values. The oxygen demand refers to the amount of oxygen that is needed to stabilize the organic content of the wastewater. The two most common oxygen demand methods of defining organic matters in wastewater are the biochemical oxygen demand and the chemical oxygen demand.

Biochemical oxygen demand

Biochemical oxygen demand (BOD) estimates the degree of organic content by measuring the oxygen required for the oxidation of organic matter by the aerobic metabolism of microbial communities. A characteristic simple carbonaceous compound is fructose, and this is oxidized as follows (Equation 1.1):

The common procedures of BOD measurements are dilution method and respirometric method.

The dilution method is the most common method in use for the waste-water industry and consists of diluting wastewater samples with a nutrient solution (to provide essential minerals for microbial activities), according to wastewater strength, within airtight bottles that are also saturated with air (for facilitating aerobic metabolism), and measuring the dissolved oxygen at the start and periodic intervals of the analysis. A five-day period is generally engaged, and the BOD measured thereafter is called BOD5. The authorative procedures of BOD analysis can be found in the standard methods (Eaton et al., 2005). One cautionary note for BOD analysis is that it involves the degradation of organic matters by a microbial population in the testing bottles; the microbial count is important for the analysis and insufficient microbial count will underestimate the BOD. This issue is particularly critical for the food wastewater analysis because some food processing operations involve thermal processing or other sterilizations, and the wastewater generated in those operations may not have sufficient mi-crobial count for accurate analysis of BOD in the wastewater. The possible remedy for this is to measure the wastewater from those operations constantly for a long period of time or to add the adapted "seed" of bacteria to the wastewater. The dilution method is the time-honored but time-consuming method. An upgraded version of the method involves the use of a dissolved oxygen electrode in the BOD5, enabling continuous readings of the dissolved oxygen during the five-day period. Commercial products of the BOD5 analysis instruments based on the dilution method are available.

Another phenomenon that could alter the BOD analysis result, though not occurring in all food and agricultural wastewaters, is nitrification of the wastewater. Nitrification is a biochemical process of converting organic nitrogen (e.g., proteinaceous compounds) into nitrate (Liu et al., 2003). This is an aerobic process and therefore uses additional oxygen. One method of inhibiting nitrification is to use inhibitive chemicals such as allyl thiourea, methylene blue or 2-chloro-6-(trichloromethyl) pyridine (Metcalf and Eddy, Inc., 2002).

Respirometric method is an alternative to the dilution method in BOD analysis. It accelerates BOD analysis by combining biochemical processes with faster chemical reaction. The basic design of the respirometric method is the use of a continuously stirred bottle with partially filled wastewater (and a headspace), which is connected to a reservoir of alkali (usually potassium chloride) that absorbed the CO2 generated from the degradation of organic matters in the wastewater sample. The pressure changes in the headspace of the BOD bottle will be constantly monitored for consumption of O2 in the wastewater sample. Even with the hybrid BOD analysis methods, the BOD analysis is slow and unsuitable for process control purposes in a wastewater treatment plant. Another approach to measuring the organic content of wastewater, chemical oxygen demand, is developed to complement the BOD analysis.

Chemical oxygen demand

Chemical oxygen demand (COD) is an estimator of the total organic matter content of wastewaters. COD approach is based on the chemical oxidation of the organic matters in the wastewater: either oxidation of the organic matters by permanganate or oxidation by potassium dichromate (K2Cr2O7). COD analysis using dichromate is the most common method today and is used to continuously monitor biological wastewater treatment systems. The value of COD for a given wastewater stream is usually higher than that of BOD5 due to the fact that not only organic matters can be oxidized, but nonorganic matters can as well. It is common to correlate the values of COD to the values of BOD5 and use the rapid COD analysis method (about two hours) to determine the organic content of the waste-water sample. The COD test utilizes K2Cr2O7 in boiling concentrated sul phuric acid (150°C), in the presence of a silver catalyst (Ag2SO4), to facilitate the oxidation. The detailed procedures of COD test can be found in the standard methods (Eaton et al., 2005). The following reaction describes the oxidation of organic carbonaceous compounds in the presence of K2Cr2O7 and the catalyst (Equation 1.2):

The COD is calculated by titrating the remaining dichromate of known amount or by spectrophotometrically measuring the Cr3+ ion at 606 nm (or remaining Cr2O72~ at 440 nm). The titration method is, though more time-consuming, more accurate than the spectrophotometry method.

A common interference in the COD testing is chloride in the waste-water, which is readily oxidized by dichromate (Equation 1.3):

This interference that overestimates the COD level in the wastewater may be prevented with the addition of mercuric sulphate (HgSO4) to remove Cl~ in the form of an HgCl2 precipitate (Bauman, 1974). The above COD method is called the open reflux method in the standard methods (Eaton et al., 2005). Another COD testing method is called closed reflux method (Eaton et al., 2005). In this setting, the oxidation takes place in the closed tubes filled with a small wastewater sample mixed with Ag2SO4 and HgSO4. The tubes are heated to hasten the oxidation and, as a result, results in shorter times. The COD is determined spectrophoto-metrically. Several commercial designs based on this method are available in the form of an apparatus or kit with solution ampoules and pre-measured reagents.

Total organic carbon

Total organic carbon (TOC) is a method based on the combustion of organic matters in the wastewater sample to CO2 and water, dehydration of the combustion gases, and running the gases through an infrared analyzer. The analyzer reads out the amount of CO2 from the combustion, which is proportional to the amount of carbon in the wastewater sample. Sometimes, presence of inorganic carbon compounds such as carbonates and bicarbonates in the wastewater may distort TOC readings. The problem may be eliminated with purging of inert gases. Commercial TOC devices em ploy a different strategy: they have two combustion tubes to accommodate combustions of inorganic carbon compounds at 150°C and organic carbon compounds at 950°C. The necessary use of furnace in the TOC analysis renders this method more expensive, thus preventing TOC analysis from being widely used.

Nitrogen and phosphorus

The sources of nitrogen (N) and phosphorus (P) in food and agricultural wastewater may include artificial fertilizers, synthetic detergents used in cleaning food processing equipment, and metabolic compounds from pro-teinaceous materials. These elements are nutrients for microbial flora; however, if they are present in excess, they may cause proliferation of algae in the receiving water body, adversely affecting the ecological balance. Increasingly, many wastewater treatment plants employ advanced wastewater treatment technologies to reduce or eliminate the amount of N and P in the discharge.

Sampling

Accurate characterization of food and agricultural wastewater depends on accurate sampling of wastewater. Special attention should be paid to the representative sampling of a wastewater stream. Commercial sampling instruments are widely available and a simple in-house lab-scale continuous sampler can be set up with relatively modest means (Metcalf and Eddy, Inc., 2002). The procedure for a particular parameter of wastewater management may be found in the standard methods (Eaton et al., 2005).

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