Wastewater Characterization for
Evaluation of Biological Phosphorus Removal

Wastewater Fractionation

Fraction of COD in Wastewater

Before biological phosphorus removal process design models can be used, it is necessary to determine the various fractions of the influent COD. These fractions are needed to accurately describe the behavior of the biological phosphorus removal process. Figure 1 shows the subdivisions as presented by Ekama et al. (1984). Although the terminology varies, these are the same fractions used in the IAWPRC Activated Sludge Model 1 (Henze et al. 1987).

Figure 1. Division of the total influent COD in municipal wastewater into its various constituent fractions.

The first major subdivision of the total influent COD (S_ti) is into biodegradable (S_bi) and unbiodegradable (S_ui) fractions. Each of these is further subdivided. The unbiodegradable COD (S_ui) consists of two fractions: unbiodegradable soluble COD (S_usi) and unbiodegradable particulate COD (S_upi). (See List of Select Symbols for definitions.)

S_usi will pass through the treatment process and be discharged with the effluent. S_upi is enmeshed in the activated sludge. The mass of S_upi entering the system will equal the mass leaving the system via activated sludge wasting. Thus, S_upi has the principal effect of increasing the mixed liquor suspended solid (MLSS) concentration.

The biodegradable COD fraction (S_bi) is divided into readily biodegradable soluble COD (S_bsi) and slowly biodegradable particulate COD (S_bpi). S_bsi is taken up by activated sludge in a matter of minutes and metabolized, giving rise to a high unit rate of oxygen demand for synthesis. S_bpi must first be sorbed onto the microorganisms, and broken down to simple chemical units by extracellular enzymes before finally being metabolized by the microorganisms. The soluble readily biodegradable fraction, S_bsi, plays an important role in biological phosphorus removal because phosphorus-removing microorganisms sequester volatile fatty acids (VFAs) in the S_bsi fraction, using the energy obtained from cleavage of a phosphate bond of the polyphosphates stored within the biomass.

Readily Biodegradable Soluble COD (S_bsi)

In the anaerobic zone of a BPR process, only the readily biodegradable soluble COD (S_bsi) component is susceptible to fermentation to form VFAs within the short detention time (1 - 2 hours).

Early evidence of the need for readily biodegradable substrate in phosphorus removal processes was provided by Fuhs and Chen (1975). They proposed that the enrichment of activated sludge with the phosphate accumulating bacteria, Acinetobacter, would ensure efficient biological phosphorus removal. The growth of Acinetobacter could be ensured by supplying readily biodegradable short carbon chain substrates such as ethanol, acetate, and succinate to an anaerobic zone in the process. Such a carbon source could also be provided by bleeding in fermented primary effluent or anaerobic digester supernatant liquor.

Further evidence of the need for VFAs in biological phosphorus removal was provided by Venter et al. (1978) and Osborn and Nicholls (1978). These experiments indicated that S_bsi is mostly utilized in the anaerobic reactor. This concept was also postulated by Nicholls and Osborn (1979) when they stated that S_bsi was taken up into the cell under anaerobic conditions and stored as poly-ß-hydroxybutyrate.

In seeking an explanation for the behavior of different phosphorus release patterns, Ekama et al. (1984) found that phosphorus release increased as the readily biodegradable soluble COD (S_bsi) increased. Ekama et al. (1984) concluded that a prerequisite for phosphorus release in the anaerobic zone is that the concentration of readily biodegradable soluble COD (S_bsi) surrounding the microorganisms in the anaerobic zone must exceed approximately 25 mg/L. Therefore, S_bsi is thought to be a very important wastewater characteristic in the process of biological phosphorus removal.

Determination of the COD Fractions

Biodegradable COD (S_bi) Determination

Theory:

Biodegradable COD (S_bi) may be determined using the total biological demand (T_bOD) concept of Mullis and Schroeder (1971). The T_bOD concept assumes that particulate organic materials are hydrolyzed when the biological oxidation process is completed (normally after 24 hours). This was true in tests performed on wastewaters from several municipalities during this study. Thus, T_bOD is conceptually equal to the biodegradable COD including the soluble readily degradable COD (S_bsi) and the particulate slowly degradable COD (S_bpi). Using T_bOD as the value for S_bi is thought to be adequate for design.

T_bOD can be determined in a batch test simultaneously with the yield coefficient, Y, as described in the section on "Y and k_d Determination by Batch Test." The batch test should be conducted under similar operational conditions of the wastewater treatment plant of interest, including sludge age, food to microorganism ratio (F/M), mixed liquor suspended solid (MLSS) concentration, etc. A worksheet for determination of T_bOD and Y is provided in Table 3. Currently we are trying to develop a simpler method using an electrolytic respirometer.

Apparatus:

A 10 L bottle (reactor)
Diffuser
0.45 m glass fiber filter, beakers, pipettes
COD measurement apparatus
VSS measurement apparatus
Filtration apparatus

Table 3: Worksheet for T_bOD and Y Determination.

Procedure:

The batch test procedure to determine T_bOD (S_bi) consists of the following steps:

1. Obtain 8 L of composite wastewater sample.

2. Measure the initial total COD and initial soluble COD (the COD of filtrate passing through a 0.45 m filter, COD_f) of the wastewater sample. The COD of the wastewater suspended solids is obtained by subtracting soluble COD from total COD.

3. Obtain 8 L of acclimated activated sludge.

4. Place a portion of the wastewater and activated sludge into an 8 L reactor. The dilution ratio used can be the same as the F/M ratio¹ at the treatment plant of interest. For example, the Ashland wastewater treatment plant has the F/M ratio of 0.67; thus, 1.3 L of activated sludge with VSS of 1,840 mg/L can be mixed with 6.7 L of raw sewage with BOD5 of 240 mg/L to obtain the F/M ratio of 0.67 in an 8 L reactor.

5. Aerate the reactor to reach a dissolved oxygen level of approximately 2 mg/L. If an air pump with a diffuser does not provide sufficient mixing, add a mechanical mixer. The mixture is aerated for 24 hours, and samples are taken periodically.

6. Measure the COD of the mixture (COD_m) and the filtrate passing through a 0.45 m filter (COD_f). Duplicate or triplicate sample analyses is recommended. The COD of the suspended solids is calculated by subtracting COD_f from COD_m.

7. For Y and k_d determination (described in the section on "Y and k_d Determination by Batch Test"), measure the VSS of the mixture and COD of the filtrate passing through a 0.45 m filter (COD_f) at 0, 1, 2, 3, 4, 5, 6, 12, 18, and 24 hours.

Data Analysis:

T_bOD is the difference between the initial substrate COD and the final unbiodegradable substrate COD in the reactor:

T_bOD = S_bi = initial substrate COD - final substrate COD (Final COD_f) ⁽¹⁾

where
initial substrate COD = initial COD_m - initial biomass COD; and
initial biomass COD = initial mixture suspended solids COD - raw wastewater suspended solids COD

Because the wastewater sample is diluted by adding activated sludge to the reactor, the actual T_bOD is obtained by adjusting the test T_bOD by the dilution factor. An example calculation of T_bOD, using data from Table 4, is provided below.

Calculation of T_bOD from Example Test Data:

1. Use measured total COD and soluble (filtered) COD of wastewater sample (see Table 5) to calculate the wastewater suspended solids COD (SS COD_w).
SS COD_w = total wastewater COD - soluble wastewater COD.
SS COD_w = 488 - 203 = 285 mg/L.
2. Use measured initial total COD of mixture (COD_m) and soluble (filtered) COD of mixture (COD_s) (see Table 4) to calculate initial suspended solids COD of the mixture (SS COD_m).
SS COD_m = initial COD_m - initial COD_s.
SS COD_m = 792 - 153 = 639 mg/L.
3. Calculate the mixture biomass COD as follows:
Mixture biomass COD = SS COD_m - SS COD_w.
Mixture biomass COD = 639 - 285 = 354 mg/L.
4. Calculate the initial mixture substrate COD as follows:
Initial mixture substrate COD = initial COD_m - mixture biomass COD.
Initial mixture substrate = 792 - 354 = 438 mg/L.

5. The final substrate COD of mixture is the measured final soluble (filtered) COD of the mixture. Therefore the test T_bOD is calculated as follows:
   Test T_bOD = initial mixture substrate COD - final mixture COD_s.
   Test TbOD = 438 - 71 = 367 mg/L.
6. The test T_bOD must be adjusted by the dilution ratio to obtain wastewater T_bOD as follows:
   Wastewater T_bOD = test T_bOD x (volume of mixture/volume of wastewater).
   Wastewater T_bOD = 367 x (8/6.7) = 438 mg/L.

Personhours needed: 30 hours + acclimation time (0-30 hours depending on wastewater).

Soluble Readily Biodegradable COD (S_bsi) and Soluble Unbiodegradable COD (S_usi) Determination

Mamais et al. (1993) developed a rapid physical-chemical method for determining the soluble readily biodegradable COD (S_bsi) and the soluble unbiodegradable COD (S_usi). Flocculation, precipitation, and filtration of wastewater samples allow for the direct measurement of S_bsi and S_usi.

The method is based on the assumption that the influent unbiodegradable soluble COD (S_usi) is equal to the truly soluble effluent COD from an activated sludge plant treating the wastewater with a sludge age > 3 days. Flocculation and precipitation of the samples removes colloidal material that normally passes through a 0.45 m membrane filter.

Thus,
S_bsi = (total truly soluble COD_inf) - (soluble unbiodegradable COD, S_usi). ⁽²⁾

The total truly soluble COD of the raw wastewater is determined by flocculating the wastewater influent with Zn(OH)₂ at pH = 10.5, filtering with a 0.45 m filter, and then measuring the COD of the filtrate. The unbiodegradable soluble COD (S_usi) is determined by performing the above test with the effluent under the same assumption described above. Subtracting S_usi from the total soluble COD of the raw wastewater yields the influent soluble biodegradable COD fraction (S_bsi).

Apparatus:

Magnetic stirrer, stirring bar, and pH meter
0.45 m glass fiber filter, beakers, pipettes
Filtration apparatus
VSS measurement apparatus
COD measurement apparatus

Procedure:

A detailed flocculation method is described as follows:

1. Add 1 ml of a 100 g/L zinc sulfate solution to a 100 ml wastewater sample and mix vigorously with a magnetic stirrer for 1 minute.
2. Adjust the pH to approximately 10.5 with 6 Molar sodium hydroxide solution (NaOH).
3. Settle quiescently for a few minutes.
4. Withdraw clear supernatant (20 - 30 ml) with a pipette and pass through a 0.45 m membrane filter.
5. Measure COD of the filtrate.

Particulate Slowly Biodegradable COD (S_bpi) and Particulate Unbiodegradable COD (S_upi) Determination

From the influent biodegradable COD (S_bi) (determined by the T_bOD method) and the influent soluble readily biodegradable COD (S_bsi), the particulate slowly biodegradable COD (S_bpi) can be obtained:

Finally the particulate unbiodegradable COD (S_upi) is obtained by:

Once the values of S_bsi and S_usi are obtained together with T_bOD (or S_bi), the COD fractionation of wastewater is completed. Table 6 lists the typical COD fraction in municipal wastewater. Table 7 provides actual test data from the Ashland wastewater treatment plant.

Fraction of Nitrogen in Wastewater

In untreated domestic wastewater, nitrogen will be found primarily in the form of organic and ammonium nitrogen (NH₄⁺-N). Little (< 1%) ammonia nitrogen (NH₃-N) exists in a normal domestic wastewater with a pH of 7. A typical total nitrogen in domestic wastewater consists of about 60% ammonium nitrogen and 40% organic nitrogen with less than 1% in the form of nitrate and/or nitrite.

Analytically organic nitrogen and total ammonia nitrogen are measured simultaneously with total Kjeldahl nitrogen (TKN). If nitrate or nitrite is present in the wastewater, the TKN test will not include them. Table 8 gives a range of typical nitrogen concentrations found in untreated domestic wastewater.

Nitrogen transformations that can occur in biological treatment systems are shown in Figure 2. Organic nitrogen can be converted to ammonium through bacterial decomposition and hydrolysis of urea. Nitrification is the process whereby ammonium is oxidized to nitrate by 2 different genera of microorganisms, Nitrosomonas and Nitrobacter. Denitrification is the process that transforms nitrates to nitrogen gas by denitrifying microorganisms in the absence of oxygen. Denitrification requires an organic carbon source. Biological treatment systems can be designed to nitrify and denitrify by providing the proper conditions for the nitrifying and denitrifying microorganisms. TKN, NH₃+NH₄⁺-N, and NO₂^-+NO₃^--N can be analyzed according to Standard Methods (American Public Health Association 1995).

As with COD, nitrogenous material can also be subdivided into fractions as shown in Figure 3. It is difficult to fractionate organically bound nitrogen into biodegradable and unbiodegradable soluble and particulate fractions, N_oi, N_ui, and N_pi. Ekama et al. (1984) suggested that each fraction can be estimated only by comparing the observed response of laboratory scale processes with that predicted by the theoretical model. Although it is necessary to fractionate N_ui and N_pi for better data fitting, it is not critical to have accurate estimates since these fractions are small. Therefore, the unbiodegradable particulate organic nitrogen, N_pi, is simply expressed as 10% of the unbiodegradable particulate volatile solids in the influent, i.e., 0.1 S_upi/1.48 (or 1.42) (see "Advantage of using COD over BOD"). The unbiodegradable soluble organic nitrogen, N_ui is reported to be 0.00 - 0.04 of TKN for raw wastewater and 0.00 - 0.05 of TKN for settled wastewater (Ekama et al. 1984). Therefore, biodegradable organic nitrogen (N_oi) can be obtained by subtracting N_ai, N_ui, and N_pi from N_ti.

Figure 2. Nitrogen transformation in biological treatment processes (Sedlak 1991).

Figure 3. Fractions of nitrogen in wastewater.

Nitrate is the product of nitrification (see Figure 2). Domestic sewage without agricultural runoff normally contains little nitrate. In the aeration basin, nitrifiers oxidize ammonium and form nitrate. Since nitrifiers cannot compete with heterotrophic microorganisms in consuming oxygen, they grow in the latter part of the oxidation basin where little organic substance is present and heterotrophic microorganisms are depressed. Because the maximum growth rate for Nitrobacter is much higher than the maximum growth rate of Nitrosomonas, very little nitrite is normally present in a biological treatment system. If nitrite accumulates in a treatment plant it may be the result of toxicity to Nitrobacter. A nitrifying wastewater treatment plant contains nitrate in its effluent. If nitrifiers oxidize all the ammonia and ammonium nitrogen in the sewage influent, the effluent will contain up to 20 - 30 mg NO₃^--nitrogen/L (N/L).

The generally accepted theory for biological phosphorus removal is that sequential anaerobic-aerobic contacting processes result in selection of phosphorus-removing microorganisms. Nitrate can be introduced into the anaerobic zone by returned activated sludge from final clarifiers (in the case of a conventional activated sludge treatment plant) or by direct circulation flow (in the case of an oxidation ditch). The introduction of nitrite and nitrate depletes the readily biodegradable substrate (S_bsi), which is necessary for the phosphorus-removing microorganisms. Therefore, the presence of nitrates in the recycled stream significantly reduces the biological phosphorus removal potential.

The amount of biodegradable substrate that may be depleted due to the introduction of nitrate may be calculated as follows:

1. Calculate how much of COD will go to cell production.
2. Calculate the amount of oxygen (COD) that will go to oxidation of the substrate (how much COD is left after cell production).
3. Calculate how much of oxygen needed for step 2 will be supplied by NO₃^-.

Assuming Y is 0.45 mg VSS/mg total COD and the oxygen equivalent of the biological VSS is 1.48 mg O₂/mg VSS, the fraction of total COD that goes to cell production can be estimated as follows:

Fraction of total COD to cell mass

Thus, the oxygen used for oxidation

Since 1 mg NO₃^--N is equal to 2.86 mg O₂ from half-cell-reactions for denitrification, the nitrate-nitrogen used to supply an equivalent amount of oxygen

This implies that 8.56 mg total COD may be used for each mg of NO₃^--N added to the anaerobic zone. When Y = 0.30 mg VSS/mg total COD, 5.14 mg total COD will be used for each mg of NO₃^--N added to the anaerobic zone. Since denitrifiers use readily biodegradable substrate (S_bsi) more efficiently than phosphorus-removing microorganisms, denitrifiers have the potential to consume 5 to 9 mg total COD/mg NO₃^--N and deplete the readily biodegradable substrate (S_bsi) necessary for phosphorus-removing microorganisms.

Fraction of Phosphorus in Wastewater

Phosphorus is found in wastewater as phosphates. These can be categorized by physical (dissolved and particulate fractions) and chemical (orthophosphate, condensed phosphate, and organic phosphate fractions) characteristics. Orthophosphates applied to agricultural or residential cultivated land as fertilizers are carried into surface waters with storm runoff. Small amounts of certain condensed phosphates (pyro-, meta-, and other polyphosphates) are added to some water supplies during treatment. Organic phosphates are contributed to sewage by body wastes and food residues. Typical concentrations for various forms of phosphorus in raw wastewater in the United States are summarized in Table 9.

In the activated sludge process, condensed and organically bound phosphorus in the influent will be converted to orthophosphate. Phosphorus is removed from the process through activated sludge wasting. Thus, total phosphorus in the effluent will be primarily orthophosphate, although there will be some organic phosphorus contained in any effluent suspended solids. The fraction of phosphorus in domestic wastewater is shown in Figure 4. Phosphate fractions can be analyzed by the Standard Methods (American Public Health Association 1995).

Figure 4. Fractions of phosphorus in domestic wastewater.

More information on this topic: Gerry Novotny