Wastewater Characterization for Evaluation of
Biological Phosphorus Removal

Wastewater Sampling, Preservation, and Analysis Methods

Sampling

Sampling is an extremely important consideration in properly characterizing wastewater for biological phosphorus removal. Flow rate and wastewater quality change continuously, and these changes may affect the ability of a wastewater treatment plant to achieve consistent biological phosphorus removal. Obtaining samples that will actually represent the wastewater flow throughout the months and years to come is difficult at best. Diurnal fluctuations occur in concentration and flow volume; seasonal fluctuations occur in concentration, flow volume, and temperature; and industrial contributions to the collection system may cause wastewater characteristics to change on a short- or long- term basis. Given the variable nature of wastewater and the necessity of attaining consistent phosphorus removal, it may be necessary to collect samples that will represent "average" characteristics and approximate characteristics under more extreme conditions.

A desirable sampling method is to collect a 3-4 hour composite sample. This will provide data that may be considered representative of average wastewater characteristics throughout the day while minimizing the sample holding time. A careful review of flow monitoring records and reports generated by a facility over the past couple of years will also be helpful in assessing the seasonal characteristics of the wastewater throughout the year. If records reveal a wastewater that is highly variable in flow volume and concentration, further analysis may be required. It is not unusual to find that a particular facility may remove an adequate amount of phosphorus biologically during certain times of the year, with chemical precipitation being required during times when the wastewater characteristics are not as conducive to biological removal.

Preservation

Once a sample is taken, the constituents of the sample should be maintained in the same condition as when collected. When it is not possible to analyze collected samples immediately, samples should be preserved properly. Biological activity such as microbial respiration, chemical activity such as precipitation or pH change, and physical activity such as aeration or high temperature must be kept to a minimum. Methods of preservation include cooling, pH control, and chemical addition. Freezing is usually not recommended. The length of time that a constituent in wastewater will remain stable is related to the character of the constituent and the preservation method used. The Handbook for Sampling and Sample Preservation of Water and Wastewater (Environmental Protection Agency 1982) provides detailed guidelines on this topic. These are summarized in Table 1.

Analysis Method

The National Pollutant Discharge Elimination System permit for each municipal treatment plant dictates effluent limitations and monitoring requirements for that particular plant. For evaluating plant performance regardless of size, biochemical oxygen demand (BOD), total suspended solids (TSS), pH, and flow should be routinely monitored.

Secondary analyses may include total coliform, fecal coliform, temperature, dissolved oxygen, total volatile solids, total solids, settleable solids, nitrogen, phosphorus, chlorine residual, dissolved solids, alkalinity, metals, COD, oil and grease, and organic priority pollutants as required.

Since COD is a better energy measurement than BOD₅ (the 5-day BOD test) for monitoring carbonaceous energy removal (Ekama et al. 1984), it is recommended that COD be analyzed on a routine basis in plants designed to remove phosphorus. In addition, if a plant is designed to remove phosphorus, phosphorus (total phosphorus and orthophosphate) and nitrogen (ammonium, nitrite, and nitrate nitrogen) need to be monitored more frequently in each basin of the treatment process. The recommended routine analytical methods are summarized in Table 2.

Table 1. Required containers, preservation techniques, and holding times.^a

Table 2. Analytical Methods.

Advantage of Using COD over BOD

The BOD and COD tests are currently employed to measure the carbonaceous energy content of wastewater via its oxygen demand. BOD is a regulatory parameter used by the EPA to monitor water quality.

The BOD test is empirical and performed under strictly specified conditions and procedures. In the 5-day BOD test, the sample of wastewater is diluted with well-oxygenated and nutrient-containing water, and microorganisms adapted to the wastewater are introduced. The initial dissolved oxygen concentration is determined, and the sample is stored in darkness at 20^oC for 5 days. The difference in oxygen concentration between the beginning and end of the test period gives the 5-day BOD value. The BOD₅ test is intended to measure only the biochemical degradation of organic material, or "carbonaceous oxygen demand" of the sample, which results in the underestimation of the energy (in terms of oxygen demand) in the sample. In addition, since it takes 5 days to measure the BOD value, it is almost impossible to remedy any upset due to an unusual inflow into a treatment plant, making it difficult to use as an operational parameter.

Deviations in procedure or sample, such as the presence of nitrifiers in a sample, may give rise to uncertain results. Unless nitrification is suppressed by chemical additives, nitrifying organisms in the treated sample may multiply and utilize oxygen to convert NH₃ or NH₄+ to NO₃, giving an inflated value for the carbonaceous energy. If nitrification is inhibited, Standard Methods for the Examination of Water and Wastewater¹ (American Public Health Association 1995) requires that the results be reported as CBOD (carbonaceous BOD). Moreover, if the microorganisms are not acclimated to the wastewater, low BOD values may be obtained due to the existence of heavy metals or inhibitory compounds.

On the other hand, the COD test gives a measure of the total energy in terms of oxygen by oxidizing all biodegradable and unbiodegradable organic materials with an oxidizing agent such as potassium dichromate. Since ammonium is not oxidized, the test value reflects only the energy released due to oxidation of the carbonaceous compounds. COD can also be correlated to carbonaceous BOD₅ (CBOD₅ ),and the COD test takes only 2 hours so that the results can be used in the daily operation of a wastewater treatment plant.

Because the COD test oxidizes both biologically degradable and unbiodegradable organic materials, the energy available for biological action is usually overestimated. However, this does not reduce the usefulness of the test. If it is assumed that the fraction of organic material that is not oxidized in the COD test remains constant, then any change in COD between two points in the process provides an assessment (in terms of oxygen) of corresponding energy change. The change in COD then can be used to establish the kinetics of energy conversion in the process, i.e., the energy removal can be directly linked to the COD change. By contrast, BOD₅ values require a correction factor to correspond the energy changes, because the test values do not reflect the total oxygen demand. Albertson (1995) claimed that the results of using CBOD₅ data for raw wastewater and primary effluent could result in a 20-40% underdesign and concluded that CBOD₅ is an improper test for influent and settled raw wastewater.

Since the energy changes in biological reactions are reflected in the number of electrons transferred, the electron donor capacity can be measured in terms of the oxygen required to oxidize the carbonaceous matter to CO₂. Such a measurement is available through a COD test because COD can be expressed as a chemical reaction.

Another great advantage of the COD test is that it provides a direct estimate of the oxygen or energy potential of the volatile solids. Based on the average stoichiometric composition of activated sludge (C₅H₂NO₂), Eckenfelder and Weston (1956) calculated the theoretical mass of oxygen necessary to oxidize the mass of hydrogen ions per unit of organic mass: 1 mg of volatile suspended solids (VSS) is equivalent to 1.42 mg of O₂ or 1.42 mg of COD. Therefore, the COD/VSS ratio, f_cv, is 1.42. In the absence of more conclusive data, the COD/VSS ratio of 1.42 is generally accepted. However, Ekama et al. (1984) recommended 1.48 for the COD/VSS ratio based on the actual measurement. This relationship between VSS and COD is of greatest use when investigating the kinetics of the activated sludge process. It allows an estimation of the mass balance between the daily energy entering the plant and that leaving via the activated sludge wasted and the effluent.

  ¹Hereinafter Standard Methods for the Examination of Water and Wastewater is referred to as Standard Methods.

More information on this topic: Gerry Novotny