Critical factors affecting biological phosphorus removal ... - Springer Link

Environmental Engineering

KSCE Journal of Civil Engineering Vol. 12, No. 2/March 2008/pp. 99~107 DOI: 10.1007/s12205-008-0099-8

Critical Factors Affecting Biological Phosphorus Removal in Dairy Wastewater Treatment Plants By Chang Hoon Ahn* and Jae Kwang Park** ···························································································································································································································

Abstract Operational data from nine dairy wastewater plants (DWTPs) in Wisconsin were collected and analyzed to determine reasons for poor enhanced biological phosphorus removal (EBPR) efficiency. Several factors affecting EBPR performances in dairy were identified. Since many dairies operate five days a week, DWTPs suffer low F/M ratios during off-days, leading to sludge bulking and unstable EBPR efficiency. The most pronounced factor affecting EBPR performance was uneven organic loading caused by lack of an equalization tank or too a small volume to alleviate the fluctuation in flow and organic loading. The other factor was imbalance of nutrients, especially nitrogen. The other factors include sudden change of pH in a matter of hours and higher temperature (> 30ºC) in the summer. The high temperature was thought to shift microbial population and thus lead to the loss of EBPR capability. Unexpected discharge of cleaning solution was another problem causing poor EBPR and COD removal efficiencies. Laboratoryscale tests confirmed the effects of the COD/P ratio (organic loading) and pH on EBPR efficiency. Microscopic examination showed the presence of tetrad-arranged coccoid cells, called G-bacteria in five out of nine DWTPs. Rhodocyclus-related PAOs were also detected from Fluorescent in situ hybridization (FISH) analysis. Keywords: dairy wastewater, biological phosphorus removal (BPR), COD/P ratio, equalization tank, pH

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1. Introduction Dairy is one of the main industries in the upper Midwestern United States. The wastewater generated in dairy plants usually contains a high concentration of organic compounds such as proteins and organic acids (Danalewich et al., 1998). Since dairy wastewater has high organic strength, commonly over 3,000 mg COD/L, anaerobic or biofilm processes have been applied to dairy wastewater treatment plants (DWTPs) (Orhon et al., 1993; Ozturk et al., 1993; Rusten et al., 1993; Kasapgil et al., 1994). Other biological processes used frequently in dairies include sequencing batch reactor (SBR), oxidation ditch (OD), and extended aeration activated sludge (EAS). Especially, SBR is one of the most common processes in treating dairy wastewater for small facilities (Garrido et al., 2001; Ky et al., 2001; Torrijos et al., 2001). Raw dairy wastewater poses unique challenges for biological nutrient removal because it contains nitric and phosphoric acids used as cleaning detergents as well as high organic substances (Danalewich et al., 1998; Ahn et al., 2007). Although enhanced biological phosphorus removal (EBPR) has been considered as one of the most cost effective strategies, its performance in DWTPs was poor or failed without good explanation. Most

municipal wastewater treatment plants (MWTPs) are stable in terms of biological phosphorus removal (BPR), whereas DWTPs are suffering unstable and poor performances. It appears that DWTPs have been designed and operated under the specifications of MWTPs without considering unique properties of raw dairy wastewater. Unfortunately, no systematic analyses of the causes have been attempted yet. Danalewich et al. (1998) investigated the operating conditions of 15 dairy wastewater facilities in the upper Midwest. However, detailed description of EBPR performances was not addressed. Incomplete understanding of biological activity in raw dairy wastewater could be one of the reasons. In EBPR processes, phosphorus accumulating organisms (PAOs) are involved in phosphorus removal (Mino et al., 1998). In addition, another type of heterotrophs, called glycogen accumulating organisms (GAOs) (Liu et al., 1997), can take up organic substrate anaerobically without phosphorus (P) release and is believed to be related to the deterioration of EBPR systems. Many studies reported on major factors in competition between PAOs and GAOs, such as high COD/P ratio (Liu et al., 1997), pH in the anaerobic condition (Filipe et al., 2001), and temperature (Whang and Park, 2002). Therefore, it was necessary to identify critical factors affecting EBPR for dairy wastewater.

*Researcher Associate, Center for Environmental Biotechnology, Biodesign Institute at Arizona State University, Tempe, AZ 85287-5701, USA (E-mail: [email protected]) **Professor, Department of Civil and Environmental Engineering, University of Wisconsin, Madison, WI 53706-1691, USA (Corresponding Author, E-mail: [email protected]) Vol. 12, No. 2 / March 2008

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Chang Hoon Ahn and Jae Kwang Park

The objectives of this study were to monitor performances of nine dairy wastewater facilities, to determine major factors affecting EBPR, and to provide guidance for achieving stable and efficient EBPR.

1998). Non-soluble phosphate (Pns) was estimated by subtracting phosphate in bulk solution from TP (Schuler and Jenkins, 2003). COD, ammonia (NH4+), and nitrate (NO3–) were measured using Hach Kit (DR 4000, Loveland, CO).

2. Method and Materials

3. Field Data Analyses

2.1 Field Data Collection Nine dairy sites in Wisconsin were selected based on treatment processes: four sequencing batch reactors (SBRs), three continuous-flow extended activated sludge (EAS) processes, and two oxidation ditches (OD) modified for EBPR (Table 1). Operators at each dairy site collected the following data: flow rate, pH, water temperature, and water quality parameters, such as influent and effluent COD, total Kjeldahl nitrogen (TKN), ammonia, nitrate, and total phosphorus (TP). In Site #6, the sludge volume index (SVI) was monitored to evaluate the relationship between effluent P concentration and sludge settleability.

3.1 Configuration of Dairy Wastewater Treatment Facilities Dairy plants produce wastewater on daily basis. Production usually takes place 24 hours per day, 7 days a week with 24-hour shifts. Manufacturing activities result in a discharge of treated process wastewater and evaporator condensate. Half of the process wastewater is composed of milk truck washing water and cooling water from refrigeration process. The other half of the process is composed of evaporated water, reverse osmosis (R/O) water from milk and whey treating, and sanitary wastewater from plant cleaning. Usually, wash water is discharged twice, in the morning and in the afternoon. The process wastewater and sanitary wastewater are segregated. An equalization (EQ) tank is capable of buffering the toxicity of undesirable chemicals before entering into a biological treatment process, if properly designed. However, three out of nine sites were not equipped with an EQ tank. Moreover, the size of installed EQ tank was not sufficient to hold one day worth flow. Flow rates ranged from 0.06 (230) to 0.73 MGD (2,800 m3/d). The size and hydraulic retention time (HRT) of EQ tanks in surveyed sites are summarized in Table 1. Process wastewater is segregated into two separate wastewater streams. Normal strength wastewater consisting of wash waters, condensate, and process by-product waste stream is discharged through the plant drain system to a wastewater treatment plant. High strength wastewater (HSW) is collected separately from several manufacturing operations and is conveyed to a storage tank to minimize organic loading on the DWTP. On occasion, some of the normal strength wastewater is automatically diverted to the HSW storage tank based on refractive index, pH readings, or on-line total organic carbon (TOC) analyzer result. A portion of the HSW may be directed back to the industrial wastewater treatment facility. Waste activated sludge (WAS) is also pumped into the storage tank and then thickened in the dissolved air flotation (DAF) unit. Float from the influent DAF unit is also pumped to the HSW storage tank. HSW in the storage tank is routinely land-spread. The HRT and size of anaerobic and anoxic tanks for continuousflow dairy wastewater treatment plants are summarized in Table 2. To achieve stable biological nutrient removal, anaerobic and anoxic zones are installed in front of a main aeration basin for continuous flow processes. The HRT of the anaerobic zone ranged from 0.9 to 6.3 hours to encourage EBPR in a short period of time and to suppress filamentous bulking as a selector. Site #1 has two 55-m3 anaerobic tanks in series. Site #2 is configured with an A2/O oxidation ditch process, whereas Sites #4 and #5 are configured with an A/O activated sludge process. Site #6 has an A/O oxidation ditch process with a reaeration tank between an anaerobic tank and oxidation ditch. However, the dissolved

2.2 Biological Phosphorus Removal (BPR) Potential Test The test procedure involves mixing equal portions of activated sludge and raw wastewater (Park et al., 1999; 2001). Before conducting tests, 1 liter of mixed liquor suspended solids (MLSS) was allowed to settle for 30 minutes and 500 ml of settled sludge was used for the test. The BPR potential test was conducted in a 1-liter reactor by adding 500 ml of concentrated mixed liquor and 500 ml of raw wastewater on a magnetic stirrer. After two hours of anaerobic contact time, air was injected into the reactor using a fine bubble air diffuser for three hours. Filtered samples were obtained in a preset time interval and the subsequent release and uptake of phosphates were monitored. 2.3 Analytical Methods MLSS, mixed liquor volatile suspended solids (MLVSS), COD, nitrate (NO3–), TP, and phosphate (PO43–) were measured in accordance with the Standard Methods (APHA/AWWA/WPCF, Table 1. Size and HRT of EQ Tanks in Surveyed Sites of Nine Dairy Wastewater Treatment Plants in Wisconsina

a

Dairy plant

Processb

Flow ratec (MGD)

EQ tank size (m3)

EQ HRT (days)

Site #1 Site #2 Site #3 Site #4 Site #5 Site #6 Site #7 Site #8 Site #9

EAS (A/O) OD (A2/O) SBR EAS (A/O) EAS (A/O) OD (A/O) SBR SBR SBR

1,022 (76)2 2,763 (227) 454 (38)2 1,287 (151) 757 (38)2 833 (38)2 606 (38)2 303 (38)2 227 (151)

413 511 397 341 2u190 114

0.46 1.08 0.53 0.68 0.12 0.50

Numbers are yearly mean values (2001). OD: oxidation ditch; EAS: extended activated sludge process; SBR: sequencing batch reactor (24 h); A2/O: anaerobic/anoxic/oxic; A/O: anaerobic/oxic. c Numbers in parentheses are standard deviations b

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KSCE Journal of Civil Engineering

Critical Factors Affecting Biological Phosphorus Removal in Dairy Wastewater Treatment Plants Table 2. Summary oF Anaerobic/anoxic/oxic Volumes and Design HRTs for Continuous-flow Dairy Wastewater Treatment Plants Anoxic tanka

Anaerobic tank

Dairy plant

3

Oxic tank

Vol. (m )

HRT (hrs)

Vol. (m )

HRT (hrs)

Vol. (m3)

HRT (hrs)

300 234 190 268 210

2.5 0.9 3.6 3.6 6.3

N.A. 1,050 N.A. N.A. b ,2208b

N.A. 28.1 N.A. N.A. b 26.3b

25,700 23,200 10,600 21,900 27,000

248 285 204 267 211

Site #1 Site #2 Site #4 Site #5 Site #6

3

a

N.A.: not available. Reaeration tank.

b

oxygen (DO) level in the reaeration tank was always < 0.5 mg/l. 3.2 Characteristics of Wastewater at Nine Dairy Sites Table 3 summarizes the influent wastewater characteristics of nine DWTPs. Although most of dairies practice segregating HSW, mean values of influent COD were still high and ranged from 1,250 to 5,750 mg/l. Interestingly, a certain amount of COD reduction in the EQ tank was observed. Fermentative and facultative bacteria in EQ tanks appeared to have biological activity. Mean influent TKN concentrations ranged from 40 to 390 mg N/l. Due to the use of nitric acid in cleaning process, nitrate also exists in raw wastewater and ranged from 0.2 to 26 mg N/l, which might be a potential inhibition factor for EBPR process. Mean influent TP concentrations ranged from 26 to 73 mg P/l. The ratios of influent COD:TKN:TP varied considerably depending on dairies (100:1.8~12.2:0.6~3.9). It was apparent that some dairies are suffering nitrogen deficiency for biological treatment. Five out of nine sites have been adding nitrogen in the form of urea. Since they were adding urea manually, the dosage was not consistent. It was recommended that nitrogen be added automatically in accordance with the influent flow rate and nitrogen in the effluent be monitored regularly. Due to mixing process water with cooling water, most dairy wastewater temperature is above 24oC throughout the year. In the summer, the temperature often goes above 32oC, causing deterioration in EBPR. In laboratory-scale SBR operation, Whang and Park (2002) demonstrated that GAOs have higher acetate uptake rate than PAOs at high temperature (30oC) and will dominate in EBPR systems. Therefore, it is recommended that sludge age be

reduced to < 10 days, especially in the summer. The size of aerobic tank and F/M ratio in nine surveyed dairies are summarized in Table 4. F/M ratios ranged from 0.06 to 0.33 g COD/g MLSS½day despite high influent COD and HRTs in the aeration tank ranged from 48 to 211 hours (Table 2), which are much longer than MWTPs. Since the F/M ratio of typical EAS, OD, and SBR processes ranges from 0.05 to 0.3 g BOD/g MLVSS½ day (Metcalf and Eddy, 2003), the ratios are acceptable for the municipal facility design. In addition, field operators preferred to maintain a higher MLSS concentration (> 3,000 to 4,000 mg/l) as active sludge storage in case of system upset. These practices can avoid high organic loading in aeration basins, but larger size tanks need more aeration. Applying a typical value of F/M ratio of MWTPs may not be practical in designing DWPTs. Table 4. Size of aerobic tank and F/M ratios in nine surveyed dairies in Wisconsina Dairy

Volume (m3)

HRT (hrs)

MLSS (mg/l)

F/M (g COD/g MLSS·day)

SRT (days)


5,678 3,180 ,2 511 10,600 1,893 6,981 4,179 2,120 1,708

48 85 27 204 67 211 204 206 180

4,876 4,433 3,970 3,483 3,155 2,513 3,614 4,542 3,581

0.33 0.17 0.02 0.15 0.11 0.17 0.06 0.14 0.10

20 12 24 19 19 31 42 36 16

a

Numbers are yearly mean values (2001).

Table 3. Characteristics of Influent Wastewater at Nine Dairy Wastewater Treatment Plants in Wisconsina Dairy plant Site #1 Site #2 Site #3 Site #4 Site #5 Site #6 Site #7 Site #8 Site #9

pHin b

N.R. 8.2 7.6 4.2 7.5 7.7 7.4 9.4 7.5

Temp (oC)

CODin (mg/L)

19.5 (3.2) 32.9 27.3 (3.2) 29.9 25.8 (1.9) 34.0 (1.1) N.R. 25.3 (1.8) N.R.

1,250 (355)2, 4,890 (3,196) 3,013 (409)2, 5,754 (1,490) 1,845 (390)2, 3,106 (746)2, 1,743 (557)2, 4,720 (517)2, 2,040 (890)2,

TKNin (mg N/l) 296 (53) 149 284 (5) 186 239 (12) 256 (20) 213 (76) 385 (39) 133 (22)

a

Numbers are yearly mean value and standard deviation (2001). N.R.: no record.

b

Vol. 12, No. 2 / March 2008

101

NO3in (mg N/l)

TPin (mg P/l)

COD:TKN:TP

16.3 (12.5) 25 26.1 2(6.4) 29 25.62 (3.1) 20.2 2(0.2) 22.52 (3.3) 25.0 22.9 (18.4)

53.6 (10.3) 72.8 56.7 (4.5) 67.6 33.4 (3.3) 40.2 (14.4) 67.5 (20.1) 26.3 (6.2) 45.0 (14.8)

100: 7.7: 4.3 100: 3.0: 1.5 100: 2.8: 1.9 100: 3.2: 1.2 100: 2.1: 1.8 100: 1.8: 1.3 100:12.2:3.9 100: 8.2: 0.6 100: 6.5: 2.2


3.3 SBR Cycle Adjustment for EBPR The size and cycle of four surveyed SBRs are summarized in Table 5. HRTs range from 27 to 206 hours. It should be noticed that all the operating cycles were selected based on their own dairy product schedules, not on maximizing EBPR. The cycle of Site #3 starts with the fill stage in which the wastewater enters the bioreactor. The fill stage consists of two steps, anoxic fill and aerobic fill. The anoxic and aerobic fills were designed to remove the nitrogen source. After fill stage, the anaerobic-anoxic-aerobic cycle repeats twice. The durations of the anaerobic and aerobic stages are 40 minutes and one hour, respectively. Following the anaerobic-anoxic-aerobic reaction, an aerobic stage is continued for additional five hours. Before the settling stage, alum is added for better sludge settling. Since many of the facilities did not have an EQ tank, they often fed twice a day routinely without considering EBPR. With twostep feeding during anaerobic-aerobic cycle in SBR, it is difficult to execute EBPR properly. If substrate still exists after the anaerobic stage, EBPR efficiency will deteriorate (Ahn et al., 2007). For instance, Site #7 practiced two feeds per day without

an anaerobic stage because of insufficient EQ tank size, showing unstable and poor EBPR efficiency. Site #7 modified the anaerobicaerobic cycle such that the feed is once a day or when the EQ tank is full. As a result of this modification, effluent P concentrations became low and stable (< ~2 mg P/l). Site #9 also changed a SBR cycle similar to Site #7 and introduced a long idle time rather than unnecessary aeration. This slight modification also resulted in good phosphorus removal similar to Site #7. 3.4 Treated Wastewater Quality at Nine Dairy Sites The water quality parameters of the treated wastewater at nine surveyed dairies are summarized in Table 6. The mean effluent COD ranged from 18 to 251 mg/l and the COD removal efficiencies for all sites were > 95%. As addressed above, some dairies were suffering nitrogen deficiency and effluent ammonia concentrations were low (0.4 to 8.5 mg N/l). The mean effluent phosphorus concentration ranged between 5.6 at Site #5 and 25.6 mg P/l at Site #1. Meanwhile, P removal efficiency varied from 42% at Site #8 to 91% at Site #2. The mean effluent nitrate concentration ranged from 0.6 to 45 mg N/l. In a continuous-flow process, it is

Table 5. SBR Sizes and Cycles for Dairy Wastewater Treatment Plants 3

Dairy

Vol. (m )

HRT (hrs)

SBR cycle

Site #3

510

27

Fill (1 hr – mixing), fill (1 hr – mixing with air), react (40 min – mixing with no air), react (1 hr – mixing with air), anaerobic react (40 min), react (5 hrs – mixing with air), alum addition with mixing (15 min), settle (1 hr), decant (3~4 hrs), and idle.

204

Before modification: fill (2 hrs – mixing with air), react (8 hrs – mixing with air), fill (2~3 hrs – mixing with air), react (4 hrs – mixing with air), settle (2 hrs), decant (2~3 hrs), and idle. After modification: fill (2~3 hrs – mixing with no air), react (6~8 hrs – mixing with air), settle (2 hrs), decant, and idle.

206

Before modification: Fill (3 hrs – mixing without air), react (3 hrs – mixing with air), fill (3 hrs – mixing with air), react (3 hrs – mixing with air), fill (3 hrs – mixing with air), react (3 hrs – mixing with air), settle (2 hrs), decant (3 hrs), and idle. After modification: Fill (2 hrs – no mixing), fill (3 hrs – mixing without air), react (3 hrs – mixing with air), react (1 hr – mixing with no air), react (3 hrs – mixing with air), react (1 hr – mixing with no air), settle (1 hr), decant (3~4 hrs), and idle.

180

Before modification: Fill (1.5 hrs – mixing without air), react (5.5 hrs – mixing with air), fill (1.5 hrs – mixing without air), react (5.5 hrs – mixing with air), settle (1 hr), decant (4 hrs), and react (0.5 hr – mixing with air). After modification: Fill (3 hrs – no mixing), fill (3 hrs – mixing without air), react (3 hrs – mixing with air), react (1 hr – mixing with no air), react (3 hrs – mixing with air), react (1 hr – mixing with no air), settle (1 hr), decant (3~4 hrs), and idle.

Site #7

Site #8

Site #9

4,180

2,120

1,710

Table 6. Treated Water Quality at Nine Dairy Wastewater Treatment Plants in Wisconsina, b Dairy plant

CODeff (mg/l)

NH4+eff (mg N/l)

NO3eff (mg N/l)

TPeff (mg P/l)

COD removal (%)

P removal (%)


54 (11) 73 38 (18) 72 40 (25) 18 (8)1 N.R. 251 56 (35)

0.4 N.R. N.R. N.R. N.R. N.R. 5.3 (3.4) 8.5 N.R.

18.1 (12.5) 12.2 10.6 (0.9) 45 12.0 (2.8) 17.0 (5.0) 12.5 (3.3) 19.4 18.0 (10.2)

25.6 1(6.7) 16.5 15.6 1(7.6) 18.0 1(5.1) 15.6 1(1.4) 18.6 1(8.0) 17.0 1(6.2) 15.2 18.7 (18.1)

95.7 98.5 98.7 98.7 97.8 99.4 94.7 97.3

52.2 91.1 72.5 73.4 83.2 78.6 89.6 42.2 58.4

a

Numbers are yearly mean value and standard deviation (2001). N.R.: no record.

b

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Critical Factors Affecting Biological Phosphorus Removal in Dairy Wastewater Treatment Plants

not favorable to mix a large amount of return activated sludge (RAS) with a high level of nitrate in the terms of maximizing EBPR. In a SBR process, it was recommended that a denitrification stage be included in the cycle. It was anticipated that the effluent P concentration would decrease further by introducing denitrification stage in the middle of a SBR cycle. 3.5 Sludge Settleability Fig. 1 represents a relationship between sludge volume index (SVI) and effluent P concentration at Site #6, which operates an A/O type OD process. It has been reported that there is a strong relationship between SVI and phosphorus content in biomass (Schuler, 1998). In case of DWTPs, as the effluent P concentration decreased, the SVI also decreased. Similarly, if a DWTP achieves a good EBPR, sludge settleability will improve significantly. Four out of nine sites surveyed are currently adding ferric chloride to improve sludge settleability. If these facilities are accomplishing a good EBPR, they could stop adding ferric chloride. 3.6 PAOs in Dairy Sludge During the study, dairy sludge samples from six sites (Sites #1, #3, #4, #6, #7, and #8) were collected seasonally and poly-P contents in biomass (PnsMLVSS) were analyzed. Fig. 2 represents seasonal variations of poly-P content in different dairy sludge. Before surveying, it was anticipated low poly-P contents in dairy sludge because of poor EBPR efficiency. However, poly-P contents in most dairy sludge samples (g P/g MLVSS) were > 5%, much greater than that of 2~3% in typical municipal activated sludge. In Site #1 sludges collected in the spring one year apart, poly-P content changed from 14% to ~6%. The lack of an EQ tank might contribute to the inconsistency of EBPR. In Site #3 sludge, poly-P content varied widely from 4 to 10%. Site #3 suffered a high pH problem due to basic solution discharge, resulting in sudden loss of poly-P in biomass. pH should be controlled before entering the SBR. Site #4 sludge had poly-P content ranged from 6 to 7.5%. Despite high poly-P content, EBPR efficiency was poor at this site. Site #6 sludge had the

Fig. 2. Seasonal Variations of Poly-P Content (PnsMLVSS) in Dairy Sludge from Six Different Dairy Wastewater Treatment Plants

highest poly-P content averaged at 8.6% and ranged from 5% to 11%, while Site #8 sludge had the least poly-P content averaged at 1.8%. Poly-P content in Site #7 sludge was relatively stable throughout the survey year and was averaged at 5.7%. Since the influent COD-P ratio is higher than 50 at most dairy sites (Table 3), it was anticipated that PAOs might lose their dominance and be washed out from the DWTPs. Fluorescent in situ hybridization (FISH) technique had been attempted to confirm the existence of PAOs in dairy sludge, but it was unsuccessful because of abundance of extracellular polymeric substances (EPS). Because of this, the activated sludge from a full-scale municipal wastewater treatment plant (Madison, Wisconsin) was acclimated with raw wastewater from a dairy site (Site #4) in a laboratory-scale SBR. Within 20 days, poly-P content in biomass increased to 9.5% (g P/g MLVSS). At this moment, FISH probe was used. FISH protocol and SBR operation are described in detail in Ahn et al. (2006, 2007). Fig. 3 shows Rhodocyclus-related PAOs in dairy sludge. The percentage of PAOs to the total bacterial cell was accounted for 38.3%. Besides, it was observed that tetrad-arranged coccoid cells, called G-bacteria, occurred in five dairy sites (Servior et al., 2003; Ahn et al., 2006).

4. Factors Affecting EBPR Performance in Dairy Sites From the analyses of full-scale data and laboratory-scale batch tests, the following factors affecting EBPR were identified: influent pH fluctuation, low/fluctuating organic loading, nutrient (specifically nitrogen) deficiency, temperature, and toxic chemical discharge.

Fig. 1. Relationship between SVI and Effluent Phosphorus Concentration in a Dairy (Site #6) Vol. 12, No. 2 / March 2008

4.1 Influent pH The pH of influent wastewater varied from 4.2 to 9.4, depending on the type of products produced in individual dairies

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Fig. 4. Phosphorus Release and Uptake at Different pHs Ranging from 6.5 to 11 Fig. 3. Fluorescent Microscopic Images (u630) of Mixed Liquor from SBR after 20 days Feeding with Raw Dairy Wastewater (Site #4): Red Spots with CY3 Labeled with PAO 462 Probe and Blue Spots with 4'-6-diamidino-2-phenylindole (DAPI) Staining

in Table 3. In the biological nutrient removal process, the maximum rate of nitrification occurs at pH 7.2~9.0 and that of denitrification occurs at pH 6.5~7.5 (Metcalf and Eddy, 2003). The pH in the anaerobic stage has been reported to play an important role in determining the dominant species between PAOs and GAOs (Filipe et al., 2001). Filipe et al. (2001) reported that a pH in the anaerobic stage above 7.25 would be favorable for the growth of PAOs. Most heterotrophs are in general unable to tolerate pH levels above 9.5 or below 4.0. A series of BPR potential test was conducted to determine the effect of wide range of influent pH in dairy wastewater. Due to the difficulty of obtaining dairy sludge with large populations of PAOs, the sludge from a typical MWTP (Nine Springs, Madison, Wisconsin) was used. Only initial pH was adjusted with 1 N NaOH or 1 N H2SO4 and the variation of pH was monitored during the test. Fig. 4 shows P release/uptake during BPR potential test with a wide range of influent pHs. Phosphorus release occurred between pH 6.5 and 8.5 but the phosphorus uptake was affected. When the initial pH was 6.5, phosphorus release was the second best, while phosphorus uptake was the greatest, which matches with the report from Liu et al. (1996). At pH t 9.5, there was little phosphorus release in the anaerobic stage and a significant release of phosphorus in the aerobic stage. pHs of the bulk solution were inclined to converge between 8 and 9 by the end of experiments. Based on experimental results, it was recommended that influent pH should be > 6.5, but < 8.5 to achieve good and stable EBPR. A sufficiently large EQ tank would be favorable in the case of the pH fluctuation. Since many dairies use acids and alkaline chemicals in turn during cleaning process, a large EQ tank will neutralize the pH, leading to less chemical requirement for pH adjustment.

4.2 Low and Fluctuating Organic Loading Most dairies operate five days a week, resulting in an insufficient organic loading during off-production days. Since flow rate and organic loading change significantly over time, DWTPs have been designed to have large aeration tanks with a high biomass concentration. Historically, this design approach was successful in achieving stable and good COD/BOD5 removal and low sludge yield in MWTPs. However, dairy sites have experienced filamentous bulking problems due to a low F/M ratio in the aeration basin and high oxygen demand during feeding or near an inlet. To prevent bulking problems, it was recommended that SBRs have one feed rather than two or three feeds per cycle so that a higher substrate gradient can be achieved (Jenkins et al., 1993). In order to implement stable EBPR, volatile fatty acids must be present in a sufficient quantity and constant level. Temmink et al. (1996) found that even with short periods of low organic loading, EBPR processes were often disturbed significantly. The disturbance was caused by a partial or complete depletion of the internally stored poly-E-hydroxybutyrate (PHB). In addition, Bradjanovic et al. (1998) demonstrated that excessive aeration would deteriorate the EBPR process due to a gradual depletion of PHB. Since many DWTPs have been designed with long HRTs (2 to 9.3 days in Table 4), it may not be feasible to achieve maximum EBPR. In order to overcome this cyclic low organic loading, it was recommended that the highest possible substrate gradient be created within the reactor by providing a large EQ tank to have at least one-day HRT and one feed per cycle or by configuring a plug-flow pattern in anaerobic/anoxic/oxic tanks. 4.3 Influent COD/P Ratio Influent COD/P ratio has been known to influence EBPR. Pitman (1991) reported that, whenever the influent COD/P ratio dropped below 50, EBPR became unstable. Also, Randall et al. (1992) showed the relationship between the influent COD/P ratio and the effluent P concentration. Although the COD/P ratio of > 35 has been recommended for stable P removal with municipal wastewater, it may not be applicable for dairy wastewater.

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Fig. 5 shows the relationship between the influent COD/P ratio and the effluent P concentration for Site #8, a whey processing facility. The raw wastewater from Site #8 suffered from high ammonia concentration and high pH problems, causing the poorest P removal among the nine surveyed dairy sites. COD/ and TP values were monitored on weekly bases at the onsite laboratory. During the study period, the mean influent COD was 4,720 mg/l and the mean influent TP concentration was 26 mg P/ l. Thus, the mean COD/P ratio could be estimated at 182 (g COD/g P), whose value is almost ten times greater than typical municipal wastewater (Randall et al., 1992). In Fig. 5, the effluent P concentration decreased as the influent COD/P ratio increased. Note that the SBR cycle was not optimized for EBPR at the surveyed period, although there was an anaerobic stage. However, the trend was rarely observed at the other sites, suggesting that there are many other factors affecting EBPR, such as initial pH, temperature, and toxic chemical spills. In addition, other reasons for poor relationship include the presence of nitrate in anaerobic zones and discharging of unidentified wastewater containing low-COD, which could render a low influent COD/P ratio. Interestingly, another dairy site showed a different aspect of the influent COD/P ratio. Site #1 diverts HSW to the anaerobic digester and the COD/P ratio of raw wastewater was low (< 15). Occasionally, when HSW was fed to the main biological treatment stream during the repair of the anaerobic digester, field operators reported that the full-scale EBPR process showed improved EBPR performance. The addition of HSW from the whey drying operation changed the COD/P ratio, leading to a favorable condition for EBPR. The mixing condition for EBPR potential test, including characteristics of raw wastewater and HSW is summarized in Table 7. As the percentage of HSW in the mixture increased by 10%, 20%, and 30%, the influent COD/P ratio increased from 12.4 to 16.2, 17.7, and 18.5, respectively. It was difficult to achieve COD/P ratio over 25 because of the nature of

Fig. 5. Relationship between Influent COD/TP Ratio and Effluent Phosphorus in a whey Processing Facility with a high Influent pH and Ammonia Concentration (Site #8) Vol. 12, No. 2 / March 2008

Table 7. COD and Total Phosphorus Concentrations of Raw Wastewater and High Strength Wastewater from a Dairy (Site #1) for Biological Phosphorus Removal Potential Test Mixing condition

COD (mg/l)

Raw wastewater 990 High strength wastewater 14,231 0% mixture 990 10% mixture 2,314 20% mixture 3,638 30% mixture 4,962

Phosphate F/M (g COD/g COD/P (mg P/l) MLSScycle) 1 80 707 180 143 206 268

12.4 38.8 12.4 16.2 17.7 18.5

0.20 0.46 0.73 0.99

raw wastewater. Addition of > 30% of HSW to raw wastewater was not considered because of high F/M ratio (> 1.0). Fig. 6 represents the effect of the influent COD/P ratio using sludge, HSW, and raw wastewater samples obtained from Site #1. As the ratio increased, the phosphorus release and uptake was enhanced, leading to better removal efficiency. When the COD/P ratio was 12.4, the phosphorus uptake was poor despite almost 50 mg/L of phosphorus release during the anaerobic stage. However, even at the ratio of 16.2, there was sufficient phosphorus uptake to lower phosphorus in the effluent. This result suggested that there is a positive relationship between the influent COD/P ratio and the effluent phosphorus concentration, even at low COD/P ratios. 4.4 Temperature Effect Most dairy wastewater temperature is above 27oC throughout the year. In general, the high wastewater temperature is caused by the discharge of condensate of whey (COW) water. In the summer, the temperature often goes above 33oC, causing deterioration of EBPR efficiency as well as COD and TSS. A few facilities have installed surface aerators to lower the temperature below 30ºC. It was noticed that a dairy site (Site #9) experienced seasonal fluctuation in EBPR; the effluent TP concentration was < 1 to 3 mg P/L during the winter (< 27oC), where as it was > 10

Fig. 6. Phosphorus Release and Uptake at Various COD/P Ratios

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mg P/L during warmer months (> 30ºC) (Whang and Park, 2002). Using a laboratory-scale anaerobic/aerobic (A/O) SBR, Whang and Park (2002) found that PAOs lost their dominancy at high temperature (30oC) and sludge age of > 10 days. After reducing the sludge age to 3 days at the same temperature, P removal efficiency improved drastically and the EBPR performance was stable. Therefore, it was inferred that a solution to the elimination of GAO dominance in EBPR processes is to reduce the sludge age. Since Site #9 disposed waste activated sludge twice a week, it was recommended to reduce the sludge age to a practically achievable level (15~18 days). This measure resulted in the effluent phosphorus concentration of < 3 mg P/L despite the fact that the average influent phosphorus concentration was 45.0 ± 14.8 mg P/L. This plant uses a chemical polishing step to lower effluent phosphorus concentration further to meet the discharge permit consistently by adding alum in a DAF unit used for liquid/solids separation. During the upset, once a chemical is added into aeration basins to meet the effluent P limit, it may not be able to regain EBPR because sufficient levels of phosphorus will not be available for PAOs in the biological reactor, especially for SBRs. If a chemical is added to the secondary clarifier, there may not be enough contact time for chemical phosphorus removal. Therefore, an additional flocculation/mixing tank may be needed between the main reactor and secondary clarifier to achieve better chemical phosphorus removal. If regulatory agencies are going to encourage EBPR for the dairy industry, more flexible and creative phosphorus regulation will be needed. 4.5 Discharge of Cleaning Agents Fig. 7 shows the changes in effluent phosphorus concentration over time after modification of the process at Site #6. After retrofitting of an oxidation ditch for EBPR, the site showed an excellent EBPR efficiency (97%). Although the effluent P concentration was < 2 mg P/l most of the time, there was an upset, where effluent P concentrations exceeded 8 mg/l. In the middle of survey, it was found that there was an upset noticed by a COD and EBPR efficiency drop whenever brine solution was discharged. Few studies have so far reported the effects of chemical spills on EBPR in DWWTs. A spill of a cleaning solution could lead to a complete loss of EBPR capability in a couple of days and it was not possible to recover EBPR for a couple of months (Fig. 7). Brine and cleaning solutions such as peroxyacetic acid (CH3COOH) should not be discharged suddenly or should be diverted to prevent the deterioration of EBPR.

5. Conclusions and Recommendations Phosphorus discharge from municipal and industrial wastewater treatment plants started to be regulated throughout the world. Since influent P concentration is five to 20 times greater in dairy wastewater than municipal wastewater, it may not be practical to use chemical phosphorus removal process solely and many dairies are advised to implement EBPR. However, incomplete

Fig. 7. Influent and Effluent Phosphorus Concentration Over Time in a Dairy Wastewater Treatment Plant (Site #6)

understanding of characteristics of dairy wastewater may hamper the further use of EBPR. In addition, even with good EBPR efficiency (> 90%), it may not be feasible to meet 1 mg P/l regardless of whether the regulatory limit is a monthly or annual average. Tetrad-arranged coccoid cells, called G-bacteria, were observed in five out of nine DWTPs, indicating potentially poor EBPR. Through the use of FISH with the sludge obtained from the laboratory-scale SBR fed with a dairy wastewater, the percentage of Rhodocyclus-related PAOs to the total bacterial cell in dairy sludge accounted for 38.3%. In general, extended activated sludge processes with the A/O configuration were unable to achieve stable and good EBPR based on filed data. It appears that SBRs are more flexible in optimizing the EBPR performance than continuous-flow activated sludge processes because anaerobic/anoxic stages can be controlled. However, low organic loading (< 0.05 g COD/g MLSS·day) and long sludge age (> 20 days) led to poor EBPR and settling problems. For maximum EBPR, the sludge age should be shortened to 5~15 days and the HRT in the aerobic stage must be reduced enough to avoid the consumption of PHB. A large EQ tank with HRT t 1 day is recommended to avoid the deterioration of EBPR caused by low organic loading. It was found that EBPR processes tend to produce better settling sludge but are susceptible to upsets caused by changes in organic loading, pH, and temperature. In order to maintain stable and acceptable EBPR of 1 mg P/l, more careful design and operation should be practiced based the results obtained in this study. Temporal and spatial feeding methods to create highest possible substrate gradient within the reactor must be practiced to ensure better EBPR and well-settled sludge. In other words, SBR with one large feed in a cycle (temporal) and plug flow reactor (spatial) should be used.

Acknowledgements This work was supported by the following dairies in Wisconsin: Adell Corp., Alto Dairy Cooperative, Grande Cheese, Inc., Foremost

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Farm USA, Mullins Cheese, Weyauwega Milk Products, Inc., and Wisconsin Dairy State Cheese, and Wisconsin Industrial and Economic Development Research Fund.

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