Calculation of sludge production from aerobic ASP based on COD and BOD5 – comparison of methods and model validation

A 5-day Biological Oxygen Demand (BOD5) is one of the parameters which are used to determine the amount of biodegradable organic substances in water environments. In dimensioning wastewater treatment plants, BOD5 is used for determining organic loading rates which enter the wastewater treatment plant and affect such parameters as surplus sludge production and oxygen demand. Conventional “static” design methods are based on using BOD5 as a representation of the amount of organic substrates in wastewater and then use empirically obtained coefficients for such parameters as biomass yield per BOD5 removed or amount of oxygen required per BOD5 oxidised to calculate surplus sludge production and oxygen demand. In contrast, new design methods taking advantage of dynamic activated sludge models (ASM) are based on COD (Chemical Oxygen Demand) fractions for determination of organic loading, oxygen demand and surplus sludge production. It has been found out during the course of work on wastewater treatment plant design and dynamic simulation that the results obtained from both approaches can vary significantly. Theoretical explanation of such behaviour is presented here and a design method on COD fractions is validated on two benchscale SBR reactors operating on two different wastewater streams. settled wastewater from two different wastewater treatment plants. Results of this model validation are a subject of this article. 2 SLUDGE PRODUCTION CALCULATION 2.1 Biological vs. Chemical Oxygen Demand BOD5 and COD tests are a measure of a relative oxygen-depletion effect of a waste contaminant and both of them have been widely adopted as a measure of organic pollution in water environments. Difference between BOD and COD is that BOD test measures an oxygen demand of biodegradable pollutants using a bioassay technique whereas the COD test measures an oxygen demand of both biodegradable and non-biodegradable pollutants by chemically oxidising a sample with potassium dichromate. For historical reasons, BOD5 is more widely used than COD despite its major drawbacks (Albertson, 1995). This has been now slightly changing with the introduction of dynamic mathematical models of biological reactions which are based entirely on COD fractions, (Henze et al., 1987, Gujer et al., 1999, Jeppsson). BOD5 testing procedure is pretty sensitive to operating conditions like temperature or initial amount of seed, has to be done on fresh or very well preserved sewage and may not predict true oxygen requirements when substrate is of a specific nature and bacterial population needs adaptation period before being able to oxidise organic substances in the sample (Shelton, 1991). Chemical Oxygen Demand (COD) has an advantage over BOD5 as the analysis doesn’t have to be performed under such strictly controlled conditions, testing procedure is much faster and additionally it gives a measure of the total energy in terms of oxygen and therefore can be used to formulate oxygen mass balance equations. This is the reason behind using COD fractions, not BOD5 as state variables in ASM models (Henze et al., 1987, Gujer et al., 1999, Jeppsson). Unfortunately COD also takes into account the amount of organics which are nonbiodegradable and therefore will not be oxidised in a wastewater treatment plant. Thus, before an assessment of oxygen demand or sludge production can be made, COD has to be split into biodegradable and non-biodegradable fractions, (Melcer, 2005), what requires an extra analysis and is a major reason behind lesser popularity of COD compared to BOD5. Deficiencies of a BOD5 test may sometimes cause problems in assessing true amount of biodegradable organics in wastewater. Wrong preservation of samples, long delays between sampling and testing and the need for biomass adaptation to substrate can cause an underestimation of BOD5 and in effect of BODultimate. This would lead to an underestimation of sludge production and oxygen demand for a biological wastewater treatment process like activated sludge. This statement is in accordance with Albertson (1995), which claimed that using carbonaceous BOD5 (CBOD5) in raw wastewater and primary effluent for plant design could result in a 20-40% underdesign and concluded that CBOD5 is an improper test for raw and settled influent wastewater. In contrast to BOD5, COD is said to give accurate predictions of total oxygen demand and once COD fractionation into soluble/ particulate and biodegradable/ unbiodegradable fractions, (Melcer, 2005) are done properly, sludge production and oxygen demand values obtained from COD should be close to reality. 2.2 Sludge production based on BOD5 and TSS Total sludge production is affected by several components: biomass growth and decay, production of cell debris from endogeneous decay and sludge production due to inorganic solids (XII) and nonbiodegradable volatile suspended solids (nbVSS) in the influent. Equation 1 describing observed sludge yield calculation method and presented underneath can be found in ATV A 131E, 1991.