2. WASTEWATER CHARACTERIZATION


2.1 Physicochemical. Parameters

2.1.1 pH
2.1.2 Solids content
2.1.3 Temperature
2.1.4 Odour

2.2 Organic Content

2.2.1 Biochemical oxygen demand
2.2.2 Chemical oxygen demand
2.2.3 Other methods for estimation of organic content
2.2.4 Relationships between estimates of the organic content
2.2.5 Oil and grease

2.3 Nitrogen and Phosphorous
2.4 Characteristics of Fish Processing Wastewaters
2.5 Sampling
2.6 Discharge Limits


As in most wastewaters, the contaminants present in fisheries wastewaters are an undefined mixture of substances, mostly organic in nature. Since a detailed analysis for each component is neither useful nor practically possible, most of the analyses give an overall measure of the degree of contamination present. In the text dig follows an estimation of organic content is discussed since this is one of the main pollution parameters.

2.1 Physicochemical Parameters

2.1.1 pH

The pH itself is not a contaminant but is important as a characterization parameter since it may reveal contamination or indicate the need for its correction before biological treatment of the wastewater. Effluents from fish processing plants are seldom acidic and are usually close to 7 or alkaline. This value is generally due to the decomposition of the proteinaceous matter and emission of ammonia compounds.

2.1.2 Solids content

Solids may be present either in dissolved or suspended form. Suspended solids are of primary concern since they are objectionable on several grounds: those that settle can do so in the wastewater ducts, reducing their capacity; or if they settle in the receiving waterbody they may affect the bottom- dwelling flora and the food chain; if they float, the light dig enters from the surface is reduced, and those that remain suspended reduce the amount of light that enters the water thereby affecting wildlife. The solids which settle are usually measured with an Imhoff cone (see Figure 2.1), in which a known amount of water (e.g., 1 litre) is placed. The solids which settle are estimated at fixed times, usually after 10 minutes and after 2 hours. The admissible amounts that can be discharged depend on each regulation, but discharge of wastewaters is usually not permitted if they contain solids winch settle after 10 minutes.

 

Figure 2. 1. Imhoff cone used to measure settling solids

The suspended solids are measured by passing a well-mixed sample through a fibreglass filter. The weight of suspended solids can be calculated by weight differences between the filter alone and the weight plus retained solids after drying.

Soluble solids can be measured by gravimetry after evaporation of the filtrate of a sample of known volume, but are generally not checked even though in effluents with a low degree of contamination they can be significant. They depend not only on the degree of contamination but also on the quality of the supply water: in one analysis of fish filleting wastewater it was found that 65 % of the total solids present in the effluent were already in the supply water (Gonzalez et al., 1983).

2.1.3 Temperature

With the exception of wastewaters from cooking and sterilization processes in a canning factory, fisheries wastewaters do not discharge wastes above ambient temperatures. The temperature of the receiving waterbody must not increase by more than 2C or 3C, since greater increases in temperature may affect the population balance and also reduce the solubility of oxygen, thereby threatening the survival of some forms of aquatic wildlife. Wastewaters from canning operations should be cooled if the receiving waterbody is not large enough to restrict the change in temperature to 3C.

2.1.4 Odour

Odours in fisheries wastewaters are caused by the decomposition of the organic matter that emits volatile amines, diamines and sometimes ammonia.

In wastewater that has become septic, the characteristic odour of hydrogen sulphide may also develop. Odours are very important in relation to the public perception and acceptance of any waste treatment plant. Although relatively harmless, they may affect people by inducing stress and nausea.

For measurement or estimation of odour, a test panel is exposed to odours diluted with clean air. The number of dilutions needed to reduce the odour intensity to its detectable limit is called as the detectable threshold odour concentration. The technique for determination of the threshold odour can be found in the Standard Methods (1989), but as it is complicated and subject to errors due to adaptation of the test subjects, subjectivity and sample modification, it is seldom used.

2.2 Organic Content

The organic content of the wastewater can be estimated in several ways. The most common are the oxygen demand methods, although organic carbon measurement may also be used. The first estimate is the amount of oxygen that will be needed to stabilize the organic content of the effluent. The two most common methods are the biochemical oxygen demand and the chemical oxygen demand, which are discussed in detail in the following paragraphs.

2.2.1 Biochemical oxygen demand

The biochemical oxygen demand, also known as BOD, estimates the degree of contamination by measuring the oxygen required for the oxidation of organic matter by the aerobic metabolism of the microbial flora. In fisheries wastewaters, this oxygen demand originates mainly from two sources: the carbonaceous compounds which are used as substrate by the aerobic micro-organisms, and the nitrogen-containing compounds which are normally present in fisheries wastewaters, such as proteins, peptides and volatile amines. The most common procedures are described below.

2.2.1.1 Dilution method

This is the most common procedure. It basically consists of diluting the wastewater (in a ratio that depends on its strength) with a nutrient (mineral salts) solution saturated with air, storing the dilution in airtight bottles, and measuring the dissolved oxygen at the start of the analysis and at periodic intervals. A five-day period is generally monitored and the BOD is then reported as BOD5. Detailed procedures for its analysis are given elsewhere (Standard Methods, 1989), and a simplified scheme is given in Table 2.1.

Table 2.1. Simplified description of the BOD5 test

  BOD TEST
1 Obtain sample and transport immediately to laboratory
2 Dilute samples with nutrient solution so that the maximum BOD is 6 mg/l
3 Inoculate with adapted bacteria (see discussion in text)
4 Fill the BOD bottles with the diluted effluent and seal them
5 Fill blank bottles with dilution water only
6 Determine by duplicate the dissolved oxygen (DO) in samples and in control at the start of test
7 Incubate samples for 5 days in dark at 20C
8 Determine by duplicate the DO in samples and in control
9 Calculate the net change in dissolved oxygen as: (change in DO of sample) (change in DO of control). The BOD5 is the net change in DO multiplied by the dilution factor used in step 2

Key points to be taken into account for a reliable BOD test will be highlighted. The first relates to the microbial population: since the BOD analysis involves biodegradation of organic matter by microbes, these must be present in the BOD bottles. In most fisheries effluents, the microbial count is such that this does not present a problem (a possible exception might be the wastewater from cooking or sterilization operations if the sample is taken either too hot or too close to the discharge from the autoclave). If the microbial count in the seed is not sufficient, the BOD may be underestimated because of the lag-time needed for the bacteria to proliferate. The same may happen if a non-adapted seed of bacteria is used. This underestimation may occur because of the extra time needed by the micro-organisms to adapt and/or grow to the extent needed to degrade the organic matter at a noticeable rate (see Figure 2.2). This is the reason why the oxygen consumption in the BOD should be followed daily, and not only at the start of the analysis but for a 5-day period. This second practice does not allow for detection of the pitfalls in the procedure mentioned above.

Figure 2.2. Typical BOD curves. Note that underestimation may occur if inappropriate seed is used

Another phenomenon that may occur is nitrification, since fisheries wastewaters contain appreciable amounts of proteinaceous compounds or products resulting from its degradation. This causes an additional oxygen consumption. Although nitrification begins to take place after five days, it may occur earlier depending on the amounts and form of nitrogen present and the composition of the microbial flora.

To determine the BOD which is caused solely by carbonaceous matter, chemicals (such as allyl thiourea, methylene blue or 2-chloro-6-(trichloromethyl) pyridine) may be added to inhibit nitrification in the BOD analysis (Metcalf and Eddy Inc., 1979). However, since nitrification will impose an oxygen demand on the receiving waterbody, it should be taken into account as part of the total oxygen demand of a wastewater.

The standard test calls for a 5-day incubation period at a constant temperature of 20C in the dark. The latter is needed to avoid algae growth that might interfere if the bottles are exposed to light during incubation.

As an alternative for highly contaminated wastes, lower incubation temperatures or shorter periods may be used, but the conditions of analysis should be clearly explained together with the results.

2.2.1.2 Alternatives to the dilution method for BOD determination

Although it is standard procedure, the dilution method involves inconveniences which have prompted the search for alternate, and simpler and more rapid procedures. Among the inconveniences are: the relatively long amount of time and the glassware needed for a reliable determination, the need to prepare several dilutions (with the consequent increase in the chance of errors) and the need for an acclimated seed.

An alternative to the dilution method is the respirometric method, which uses a bottle partially filled with the wastewater to be tested. The sample is continuously stiffed, and there is a reservoir with alkali (usually KOH). As the oxygen is consumed, CO2 is liberated by the microbes and absorbed by the KOH and the pressure varies. The variations in pressure, read by a manometer, allow for continuous reading of the oxygen demand. Another alternative is the use of an oxygen electrode within a bottle similar to that used in the dilution method. In this case, the bottle is filled with diluted sample, and the dissolved oxygen is measured continuously (see Figures 2.3 and 2.4). Commercial versions of these alternatives are available.

 

Figure 2.3. Manometric device to estimate the BOD of a wastewater 

 

Figure 2.4. Dissolved oxygen electrode assembled into a BOD bottle

One of the major inconveniences of the BOD5 is the time it requires to obtain results. Five days render it unsuitable for control purposes of a treatment plant or for quick monitoring since the residence time of the wastewater in the treatment facility is only of several hours. This is one of the main reasons why chemical oxidation tests (discussed below) were developed.

2.2.2 Chemical oxygen demand

Due to the inconveniences of the BOD5, alternative methods are applied to estimate the organic content of wastewater, the most common being the chemical oxygen demand (COD). Two methods are known, both based on the chemical oxidation of the matter present: one is the oxidation by means of permanganate (sometimes known as consumed oxygen) and oxidation by the dichromate ion. Oxidation by means of permanganate was the standard method until 1965 when it was replaced by the dichromate method.

The COD analysis, by the dichromate method, is more commonly used to control and continuously monitor wastewater treatment systems. The COD of an effluent is usually higher than the BOD5 since the number of compounds that can be chemically oxidized is greater than those that can be degraded biologically. It is also common to make a correlation of BOD5 versus COD and then use the analysis of COD as a rapid means of estimating the BOD5 of a wastewater. This may be convenient since only about three hours are needed for a COD determination, while a BOD5 takes at least 5 days. However, this procedure can be used only for specific situations where there is low variability in the composition of a wastewater, and the results of a system cannot be used reliably in other cases.

The method of COD which uses dichromate as oxidant is carried out by heating under total reflux a wastewater sample of known volume in an excess of potassium dichromate (K2Cr2O7) in presence of sulphuric Acid (H2SO4) for a fixed period (usually two hours) in presence of silver sulphate (Ag2SO4) as catalyst. The organic matter present is oxidized and, as a result, the dichromate ion (orange colour) is consumed and replaced by the chromic ion (green colour):

Cr2O72- + 14H+ + 6 e- 2Cr3+ + 7H2O

The COD is calculated by titrating the excess of dichromate or by spectrophotometrically measuring the Cr3+ ion at 606 nm. Another possibility is to measure the excess dichromate spectrophotometrically at 440 nm. Titration requires more work but is considered more precise.

The presence of silver sulphate as catalyst is needed for complete oxidation of aliphatic carbon compounds. The standard method implies cooling of the sample after the two-hour digestion period, adding a few drops of indicator (ferroin) solution and titrating the excess dichromate with a solution of ferrous ammonium sulphate of known concentration, until the colour changes from brilliant green to reddish brown. The titration reaction corresponds to the oxidation of the ferrous ammonium sulphate by the dichromate:

Cr2O72- + 14H+ + 6Fe2+ 2Cr3+ + 6Fe3+ + 7H2O

The change in colour corresponds to the formation of the complex ferrous ionphenantroline which occurs when all the dichromate ion has been reduced to Cr3 +:

(Fe(C12H8N2)3) 3+ + e (Fe(C12H8N2)3)2+
Ferric Phenantroline   Ferrous Phenantroline
(green-blue)   (reddish brown)

A common interference factor in the COD test is the presence of chlorides. If seawater is used at some point in the processing or salt brines are used for some "curing" operations, chlorides will most probably appear in the wastewater causing interference while they are oxidized by the dichromate:

Cl- + Cr2O72- + 14H+ 3 Cl2 + 2Cr3+ + 7H2O

This interference causes erroneously high values of COD which can be prevented by the addition of mercuric sulphate (HgSO4) which reacts to form mercuric chloride and precipitates:

Hg 2+ + 2 Cl- HgCl2

This complexation technique has been used with success in high salinity waters (Gonzalez, 1986, Bauman, 1974) and seems to be satisfactory provided the ratio Cl- to HgSO4 is at least 10 to 1.

The nitrite ion also exerts COD, but since it is present in very low amounts, its influence is generally neglected.

Detailed procedures for the reflux method of measuring the COD are given in the Standard Methods (1989).

Another method of the COD determination is that of the closed reflux method (Standard Methods, 1989; Gonzalez, 1986; Jirka and Carter, 1975). In this case, a small volume of sample is heated with concentrated dichromate solution in presence of silver sulphate and mercuric sulphate. The reaction takes place in culture tubes with PTTE-lined screw caps. Heating proceeds for usually shorter times, at higher temperatures than in the open reflux method, and the COD is estimated spectrophotometrically. In this case, commercial versions of the apparatus are available as well as kits with solution ampoules and pre-measured reagents.

The open reflux method is recommended for a wide range of wastes and a large sample size can be used. The closed reflux method is more economical in terms of reactants. However, in homogenizing the samples before the analysis to obtain reproductive results, care should be taken especially in samples with suspended solids as in the case of fisheries wastewaters.

2.2.3 Other methods for estimation of organic content

2.2.3.1 Total organic carbon (TOC)

This method is based on the combustion of organic matter to carbon dioxide and water. After separation of water, the combustion gases are passed through an infra-red analyzer and the response is recorded. A diagram of this system is shown below:

 

Figure 2.5. Diagram of a TOC analyzer

The signal recorded is proportional to the carbon dioxide formed which is directly proportional to the carbon concentration in the sample injected into the combustion tube. The oxidation of the sample proceeds either by heating to almost 1000C or by intense UV radiation.

Since a wastewater contains carbonates or bicarbonates that are normally present in the supply water, these may be eliminated by acidification of the sample and purging with an inert gas prior to injection. One of the commercial versions available uses a different approach to differentiate between organic and inorganic carbon: it has two combustion tubes, one working at 150C and the other at about 950C. The first oxidizes the inorganic carbon, while the second oxidizes all the carbon present. The TOC is obtained from the difference between the readings. A major inconvenience for the widespread use of the TOC analysis is the relatively high cost of the apparatus.

2.2.4 Relationships between estimates of the organic content

Since all of these procedures are based on oxidation of the substances present in the wastewater, in principle, a correlation may be established between their results.

However, extreme care must be taken when developing these correlations since each of these analyses is affected by different factors. For example, the BOD5 assay depends on many factors, such as adaptation of the seed and degree of dilution and proper pH; a minor fraction of the COD may be due to the presence of inorganic substances that reduce dichromate; there are compounds that are not degraded during the BOD5 but are chemically oxidized such as toxic substances; there are substances resistant to oxidation by dichromate such as pyridine and acetic acid (although these are not normally present in fisheries wastewaters). Despite these factors, a correlation may be developed between two or more of the indicators of organic load (see Figure 2.6). They are not applicable to situations different from that in which it was developed and, therefore, extrapolation is not reliable and should be periodically re-checked.

Figure 2.6. BOD5 versus COD relationship for raw and treated fish filleting wastewater (Gonzalez, 1985)

2.2.5 Oil and grease

The presence of oil and grease in an effluent is mainly due to the processing operations (e.g., canning) and, to a lesser extent, on the species being processed. In any case, they should be removed since they usually float and affect the oxygen transfer to the water and also objectionable from an aesthetic point of view. They may also cling to wastewater ducts and reduce their capacity in the long term. Generally, they are measured by extraction with solvent (Standard Methods, 1989).

2.3 Nitrogen and Phosphorous

Both nitrogen (N) and phosphorus (P) are of environmental concern because they are utrients, and if present in excess they may cause proliferation of algae (algal bloom) and affect the rest of the wildlife in a waterbody.

Although N and P are normally present in the fisheries wastewater, their concentration is minimal in most cases. For biological treatment, a range of N to P in the order of 5 to 1 is recommended for proper growth of the biomass (Metcalf and Eddy Inc., 1979; Eckenfelder, 1980).

Effluent

BOD

COD

Grease/oil

Total solids

Suspended solids

Ref. No.

Finfish processing (manual)

3.32 kg/t

 

0.348 kg/t

 

1.42 kg/t

1

Finfish processing (mechanic)

11.9 kg/t

 

2.48 kg/t

 

8.92 kg/t

1

Patagonian hake filleting

327-1063 mg/l

550-1250 mg/l

8.3-79.9 mg/l

   

2

Herring filleting

3428-10000 mg/l

 

 857-6000 mg/l

   

3.4

Tuna canning

6.8-20 kg/t

 

1.7-13 kg/t

 

3.8-17 kg/t

1

Sardine plant

9.22 kg/t

 

1.74 kg/t

 

5.41 kg/t

1

Blue crab plant

4.8-5.5 kg/t

 

0.21-0.3 kg/t

 

0.7-0.78 kg/t

1

Clam plant (mechanic)

5.14 kg/t

 

0.145 kg/t

 

10.2 kg/t

1

Clam plant (conventional)

18.7 kg/t

 

0.461 kg/t

 

6.35 kg/t

1

Fish meal plant

2.96 kg/t

 

0.56 kg/t

 

0.92 kg/t

1

Fishpumping water

2100-7400 mg/l

 

10-1504 mg/l

14.5-48.2 mg/l 

 

5

Fishpumping water

3050-67200 mg/l

 

1300-17200 mg/l

18.4-64.9mg/l 

 

8

Bloodwater (fishmeal plant)

23500-34000 mg/l

93000 mg/l

0%-1.92%

2.4%-6.3%

 

6.7

Stickwater (fishmeal plant)

13000-76000 mg/l

 

60-1560 mg/l

25-62 mg/l 

 

5

References: 1: Middlebrooks (1979); 2: Gonzalez (1983); 3: Sorensen (1974); 4: Herborg (1974); 5: Cuadros and Gonzalez (1991); 6: Parin et al., 1979; 7: Civit et al., 1982 8: Nemerow (1971)

2.4 Characteristics of Fish Processing Wastewaters

The degree of pollution of a wastewater will depend on several parameters of which the most important are the operation being carried out and the fish species being processed. Considering only one type of operation, the operating routine in each factory also exerts strong influence on the wastewater characteristics. Table 2.2 lists some of the ranges reported in the literature.

A particular parameter may present a wide range of numerical values. Table 2.2 emphasizes that there is no substitute for a direct determination of contamination parameters in the effluent being considered. Moreover, given the variability that may be observed within one.

2.5 Sampling

Particular attention should be paid to the representativity of samples. Since there is no general procedure applicable to all situations, the sampling programme must be designed for each situation. The location of sampling is usually made at or near the point of discharge to the receiving waterbody, but in the analyses prior to the design of a wastewater treatment, facility samples will be needed from each operation in the fish processing facility. With representativity in mind, samples should be taken more frequently when there is a large variation in flow rate, although wide variations may occur also at constant flow rate.

Commercial sampling equipment is available, although a simple continuous sampler may be constructed with regular tubing and glass carboys as shown in Figure 2.7 (Metcalf and Eddy Inc., 1979).

 

Figure 2.7. Continuous-flow sampling device (Adapted from Metcalf and Eddy Inc., 1979)

The particular procedure for sampling may vary depending on the parameter being monitored. Usually, the description of each standard technique includes recommendation for sampling (Standard Methods, 1989). If possible, samples should be analysed immediately since preservatives often interfere with the tests. In the case of fish processing wastewaters, there is no single method of sample preservation that yields satisfactory results for all cases, and all of them may be inadequate with effluents containing suspended matter. Care should be taken in blending the samples just prior to analysis since in almost all cases they will contain an amount of settleable solids. A case in which the use of preservatives is not recommended is that of the BOD5. Storage at low temperature (4C) mg be used with caution for very short periods, and chilled samples should be warmed to 20C before analysis. For COD determinations,, the samples should be collected in clean glass bottles, and can be preserved by acidification to pH 2 with concentrated sulphuric acid.

For oil and grease, a separate sample should be collected in a wide-mouth glass bottle, well rinsed to remove any trace of detergent. The samples taken for organic nitrogen determination may be preserved similarly for COD. For the determination of solids, it should be checked that the suspended matter does not adhere to the walls, and the samples should be refrigerated at 4C to prevent decomposition of biological solids. If phosphorus is to be analysed, preservation can be ensured by storing samples in well rinsed glass bottles (plastic bottles are not recommended) at -10C and adding 40 mg/l of mercuric chloride.

2.6 Discharge Limits

in order for wastewaters to be discharged into the environment, there are a number of limitations for the concentrations of pollutants but there is no single criterion. In some cases, the limit is established by taking into account where the effluent is to be discharged. This type of criterion is also known as stream standards and is thought to be the more rational, taking into account the assimilative capacity and the intended use of the receiving waterbody. Another type of criterion is known as effluent standards. In this case, the maximum concentration of a pollutant (e.g., in milligrammes/litre) or the maximum load (e.g., kilogrammes/day) is discharged into a receiving waterbody.

Table 2.3. Summary of discharge limits in the Province of Buenos Aires, Argentina (AGOS, 1990)

    Limit if discharged to:
Parameter

Unit

Sewage collector

Surface water

Open sea

Temperature

C

< 45

< 45

< 45

pH  

7 to 10

6.5 to 10

6.5 to 10

Settleable solids in 2 h

ml/l

<5

<1

<5

Settleable solids in 10 min.

ml/l

0

0

not spec.

Free chlorine

ml/l

not spec.

<0.5

<0.5

BOD5

ml/l

200

50

150

COD

ml/l

700

250

400

Solubles in ethil-ether (grease/oil)

ml/l

<100

<50

<100

Examples of these criteria are shown in Table 2.3, where the effluent standards are given in terms of maximum concentration of a pollutant allowed to be discharged to different kinds of waterbodies; and in Table 2.4, where the limits are established for fishery industries as the maximum amount of pollutants to be discharged in a single day, as established by the Environmental Protection Agency in the USA and the proposals by the World Bank.

Table 2.4: Summary of discharge limits proposed by the United States Environmental Protection Agency (EPA), 1985 and the World Bank (WB), 1984

Industry

BOD5

TSS

Oil/Grease

EPA

WB

EPA

WB

EPA

WB

Tuna

20.0

2.2

8.3

2.2

2.1

0.27

Salmon

2.7

11.0

2.6

2.8

0.31

2.8

Other finfish

1.2

4.7

3.1-3.6

4.0

1.0-43

0.85

Crabs

0.3-10

3.6

2.2-19

3.3

0.6-1.8

1.1

Shrimps

63-155

52

110-320

22

36-126

4.6

Clams and oysters

none

41

24-59

41

0.6-2.4

0.62

In all cases, the local authorities should be consulted on the policy to be applied regarding regulations on the volume and concentration of pollutants discharged.