Dissolved Substances 2221 Organic Substances

The sum of all the different dissolved organic molecules in domestic wastewater can be approximately characterized by the average molecular composition C18H19O9N (Popel 1997) with a mass of 393 g mol-1.

The concentration can be measured by chemical oxidation using potassium dichromate. For complete oxidation, the necessary oxygen and the carbon oxidized, measured as CO2, can be calculated from:

18 19 9

For wastewater with 1 mol m-3 C18H19O9N (or 393 g m-3), 17.5-32 = 560 g m-3 O2 is needed and 18-12 = 216 g m-3 CO2-C is produced.

Therefore, the oxygen consumed and the CO2-C produced can be used as characteristics of the concentration, resulting in:

Sth = 560 g m-3 COD (chemical oxygen demand)

Sth = 216 g m-3 DOC (dissolved organic carbon) for

Sth = 393 g m-3 substrate

The theoretical value Sth = 560 g m-3 COD would only be measured for a total oxidation according to Eq. (2.1). Usually, a part of the oxidized mean products cannot be completely oxidized using the standard test. If a rapid test is used, the measured COD is even lower than that of the standard test (Section 2.3.2). For the theoretical value, the standard value would follow as:

giving a rapid test value of only:

The oxygen used for biological CO2 and H2O formation as well as for bacterial growth during a 5-day batch process (Henze et al. 2002; Table 2.1) for this theoretical domestic wastewater is:

S = 280 g m-3 BOD5 (biochemical oxygen demand over 5days without nitrification).

The BOD5 is always lower than the COD, although bacteria use oxygen not only for CO2 and H2O production, but also for growth. We can attribute this to several reasons:

• Only a part of the organics in the wastewater can be used by the bacteria as a carbon and energy source.

• Some substances can only be partly used. Some products of metabolism are excreted.

• Bacteria die and form lysis products, which can only be used in part by living bacteria.

• After 5 days, some biodegradable substances are still left. Therefore, occasionally BOD10 or BOD20 are used.

The COD and DOC of pure organic substances can be calculated in the same way as shown above, giving the theoretical COD S^,. Again, the measured values Sst and Srt are lower for the reasons mentioned above. Some data have been published for a number of chemicals often used in chemical industry (Busse 1975; Pöppinghaus et al. 1994).

Of more practical importance are the dissolved organics from the processing water of specific products. Mostly, they consist of thousands of specific compounds, as discussed above for domestic wastewater. Table 2.2 presents data on the substrate concentration S measured as COD and BOD5 in processing water from the food industry. Usually the standard method is used to measure COD.

In most cases, the concentrations of COD and BOD5 are considerably higher than in domestic or municipal effluents. Therefore, anaerobic wastewater treatment processes are often preferred. This is also true for effluents from the brewing

Table 2.2 Some typical concentrations of wastewater from the food industry (Lehr- und Handbuch der Abwassertechnik 1985).

Product

S (mg L-1 COD)

S (mg L-1 BOD5)

Sugar

7500

5 000

Maize starch

17608

11543

Potato starch

7416

6333

Wheat starch

12344-18270

Rice starch

2192

1 475

Margarine

1000-2000

500-1 000

Vegetable refinement

5 000-8 000

Fruit juice

300-800

25-1 380

Slaughterhouse

2579-6 650

1 900

Fish processing

1 530-2 567

Fig. 2.3 Daily change in flow rate and concentrations (• COD, ■ DOC, A NH4-N, A Norg) at the municipal WWTP Waßmannsdorf, near Berlin, BB 12 (basin 12), 14/15 November 2001, Q0 from on-line measurements, concentrations from 2 h mixed samples.

industry (Lehr- und Handbuch der Abwassertechnik 1985). Wastewater with high loads of organics must be treated separately in other industries such as the chemical industry, oil refineries, paper and cellulose production and the pharmaceutical industry (Lehr- und Handbuch der Abwassertechnik 1985).

In contrast to the flow rate, the concentrations of organics are nearly constant over a 24-h period (Fig. 2.3).

Therefore, the changes in load at municipal treatment plants are defined approximately by their change in flow rate. In contrast, the change in the loads from industries can differ remarkably from the changes in flow rate.

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