Enhancing Dairy Lagoon Performance with High–Rate Anaerobic Digesters

ENHANCING DAIRY LAGOON PERFORMANCE WITH HIGH–RATE ANAEROBIC DIGESTERS

G. L. Hawkins, D. R. Raman, R. T. Burns, R. E. Yoder, T. L. Cross

ABSTRACT. In a two–stage study, the possibility of using high–rate anaerobic digesters to enhance the performance of dairy lagoons was explored. Four anaerobic sequencing batch reactors (ASBR) and four downflow anaerobic filters (DFAF) were tested, with two of each type operated at 255 C, and the other two at 355 C. The first stage of the experiment explored using the high–rate digesters on liquid effluent from a screw–press treating dairy manure slurry. The first–stage experiment demonstrated that settling processes, rather than biodegradation, accounted for most of the organic matter reduction in both reactor types, when operated at a 0.5–d hydraulic retention time. Specifically, settling accounted for an average of 100% of the chemical oxygen demand (COD) removal in the ASBR reactors, and for an average of 72% of the COD removal in the DFAF reactors. These results prompted the second stage of the experiment, which explored the possibility of using the reactors to remove volatile fatty acids (VFAs) from dairy lagoon supernatant. Diluted manure slurry was spiked with acetic acid (HAc) to simulate the supernatant of an overloaded lagoon. At VFA loading rates of 1.0 and 3.0 kg HAc m–3 d–1, both reactor types achieved moderate VFA removal. However, at a loading rate of 6.0 kg HAc m–3 d–1, the ASBR reactors were ineffective, whereas the DFAF reactors, at both 255 C and 355 C, removed more than 80% of the influent HAc. At a loading rate of 12 kg HAc m–3 d–1, the DFAFs achieved removal rates of 75%; removal rates dropped to 50% at the maximum loading tested, 24 kg HAc m–3 d–1. Based on these experiments, it appears that the DFAF reactors are suited to the task of rapidly removing accumulated VFA from lagoon supernatant. Keywords. ASBR, Anaerobic sequencing batch reactor, Anaerobic filter, Odor control, VFA, Dairy waste.

naerobic lagoons are used for storage and contents (Powers and Van Horn, 1998; Powers et al., 1999). treatment of waste and wastewater from In addition to odor problems, land application of unstable industrial, municipal, and agricultural sources, waste may contaminate surface waters and groundwater A with treatment occurring through settling and through the leaching and runoff of excess organic matter microbial breakdown and stabilization of organic matter in (Arceivala, 1981; Hobbs et al., 1995). the waste. If operated in accordance with recommended Assuming a constant inflow rate of wastewater, a organic loading rates, stabilization of the waste materials reduction in lagoon organic loading rate can be achieved proceeds with minimal odors (ASAE Standards, 1999; through two distinct means: by increasing the lagoon volume USDA–SCS, 1992). However, if the organic loading rate or by lowering the concentration of organic matter in the increases, the rapid conversion of complex compounds to influent. At some agricultural production operations, the intermediate fatty acids (e.g., acetic, propionic, butyric) can former option may not be viable due to either economics or lower the lagoon pH. This lower pH can in turn suppress the available land area. Based on Hernandez and Rodriguez methanogenic population, thereby causing incomplete (1992), Hawkins and Raman (1999) hypothesized that degradation of the waste, increased lagoon odors, and lowering the concentration of organic material entering the intensified odors during land application of the lagoon lagoon might be accomplished through use of a high–rate anaerobic digestion (HRAD) system. Preliminary calculations suggested that enhancing lagoon function with a high–rate bioreactor might be economically feasible and Article was submitted for review in February 2001; approved for publication by the Structures & Environment Division of ASAE in June practical, since the storage function of the lagoon would be 2001. maintained, and since the high–rate bioreactor would only The authors are Gary L. Hawkins, ASAE Student Member, function to maintain lagoon organic loading rates within Agricultural Pollution Prevention Specialist, Dept. of Biological and prescribed limits. This article reports the results of experi- Agricultural Engineering, Coastal Plain Experiment Station, University of Georgia, Tifton Campus; D. Raj Raman, ASAE Member Engineer, ments conducted to explore the possibility of enhancing Associate Professor, Robert T. Burns, ASAE Member Engineer, lagoon function with HRAD systems, in two distinct Associate Professor, and Ronald E. Yoder, ASAE Member Engineer, configurations. Professor and Head, Biosystems Engineering and Environmental Science Experiments were conducted on large, lab–scale reactors Dept., University of Tennessee, Knoxville, Tennessee; and Tim L. Cross, (25–L working volume) operated at ambient (~25°C) and Professor and Assistant Dean, Tennessee Agricultural Extension Service, ° University of Tennessee, Knoxville, Tennessee. Corresponding author: mesophilic (35 C) temperatures. Based on a report by D. Raj Raman, Biosystems Engineering and Environmental Science Dept., Hernandez and Rodriguez (1992), the initial goal was to University of Tennessee, 2506 E.J. Chapman Drive, Knoxville, TN explore the potential for these reactors to remove approxi- 37996–4531; phone: 865–974–7722; fax: 865–974–4514; e–mail: mately half the organic matter, measured as chemical oxygen [email protected]. demand (COD), in the waste when operated at retention times

Transactions of the ASAE Vol. 44(6): 1825–1831 E 2001 American Society of Agricultural Engineers ISSN 0001–2351 1825 of less than 1 d. Preliminary calculations suggested that such Table 1. Guide to reactors, showing reactor number, reactor type a reactor, placed between a solids separator and a lagoon (ASBR = anaerobic sequencing batch reactor, DFAF = down– flow anaerobic filter), and operating temperature. (fig. 1, configuration A) might be a cost–effective remedy for Reactor Reactor Operating overloaded lagoons. However, this goal proved unreachable; number type temp. (_C) the reactors achieved low organic removal rates and low methane productivity when operated at the short retention R1 ASBR 25 times considered necessary for economic viability of this R2 ASBR 35 approach. A second approach was then tried, wherein the R3 ASBR 25 potential of removing accumulated volatile fatty acids R4 ASBR 35 (VFAs) from dairy wastewater lagoons was explored. In this R5 DFAF 25 approach, the lagoon serves as a settling basin, and the reactor R6 DFAF 35 R7 DFAF 25 only receives lagoon supernatant (fig. 1, configuration B). R8 DFAF 35 This article presents results from dairy wastewater treatment experiments conducted with two types of high–rate reactors was monitored and controlled by a separate datalog- anaerobic digestion systems, the anaerobic sequencing batch ger (Model 23X, Campbell Scientific, Logan, Utah). reactor, or ASBR (Sung and Dague, 1992), and the downflow Each DFAF included ports for influent, effluent, sam- anaerobic filter, or DFAF (Wilkie and Colleran, 1989). The pling, and sludge wasting (fig. 2). Two ports in the cap of each objectives of the experiments were to: (1) determine if solids reactor allowed biogas collection and temperature probe settling in the two reactor types contributed significantly to insertion, while a port in the reactor bottom allowed sludge removal of organic matter from the wastewater, and wasting. The influent port was located 7.5 cm above the (2) compare the VFA removal rates for the ASBR and DFAF liquid level to enhance wastewater distribution across the reactor types operating on identical dairy wastewater media. A sampling port was located approximately halfway streams. between the influent and effluent ports. The effluent port was located 2.5 cm above the reactor bottom. The DFAF packing media consisted of lava rock (Moun- MATERIALS AND METHODS tain West – Colorado Aggregate Company, Alamosa, Colo.), REACTOR COMPONENTS composed primarily of aluminum, calcium, iron, magne- Eight reactors were built and operated as described in sium, and sodium silicates (Colorado Aggregate Company, Table 1. All had a working volume of 25 L (DFAF vessels had 1988). This material has a surface area to volume ratio similar a total volume of 40 L to allow for 15 L of packing media) and to that of plastic media (ca. 250 m2 m–3), is low cost (ca. 25 were constructed of 25–cm (10 in.) dia. schedule 40 PVC to 100 times cheaper than plastic media on a surface–area pipe. Liquid levels were maintained at 82 cm and 51 cm basis), and has a relatively low bulk density (600 kg m–3) above the reactor bottom in the DFAF and ASBR, respective- (Hawkins, 2000). ly. All four ASBR reactors were housed adjacent to one Each ASBR was also constructed with ports for influent, another, in one bank of reactors, and the DFAF reactors were effluent, sampling, and sludge wasting (fig. 3). The cap of housed similarly in a second bank of reactors. Each bank of each reactor contained ports for the influent, biogas collection, temperature probe insertion, and sludge depth sampling.

Biogas Four ports along the side of each reactor allowed precise control of the decanting level, and thereby the volume HRAD Lagoon Raw Solids Liquid Recycle/ decanted per draining event. ~0.5 d ~ 180 d Waste Separator Land Application HRT HRT

Biogas Influent Thermocouple Solids Configuration A. Backflush

Biogas Supernatant HRAD ~0.5 d Effluent HRT Lagoon Raw Solids Liquid Sample Ports ~ 180 d Recycle/ Waste Separator HRT Land Application

Solids Configuration B. Figure 1. Two approaches for enhancing lagoon system performance using a high–rate anaerobic digester (HRAD) operating at a short hydraulic Plate retention time (HRT). Configuration A shows the initial approach, in Sludge Bed which the HRAD treats solid–separated wastewater prior to lagoon treat- Sludge Drain ment. Configuration B shows the final approach, in which the lagoon func- tions as a hydrolysis reactor and solids separator, and lagoon supernatant Figure 2. Schematic representation of downflow anaerobic filter (DFAF). is pumped through the HRAD for rapid removal of volatile fatty acids. Not to scale.

1826 TRANSACTIONS OF THE ASAE Influent decanted. Since the time of each operation was held constant, Biogas HRT in the ASBRs was controlled through the selection of Thermocouple the decanting port height (which determined the volume removed on each decant cycle). Influent and effluent wastewater samples were collected Recycle and analyzed for COD and VFA thrice weekly during the solids accumulation test and daily during the VFA removal Effluent experiment. Influent COD samples were collected at the inlet Ports of the influent pumps. Chemical oxygen demand was Sample measured using a closed reflux micro method (method Port #8000, HACH Company, Loveland, Colo.). Volatile fatty acid concentrations were analyzed on an HP 6890 series gas Sludge chromatograph using a crosslinked polyethylene glycol Bed column, and are reported in units of mg L–1 as acetic acid Recycle (HAc). Alkalinity and pH within each reactor were moni- Sludge Drain tored to assess the reactor performance but are not reported here. Alkalinity and pH measurements were performed in Figure 3. Schematic representation of anaerobic sequencing batch reac- accordance with Standard Methods for Examination of Water tor (ASBR). Not to scale. and Wastewater (APHA, 1998). Statistical analysis of the data was performed using SAS PROC MIXED (SAS, 1999). Reactor temperatures were monitored with type–T ther- mocouples. Optically isolated relays (Model OAC5, Opto22, REACTOR HISTORY Temecula, Cal.) enabled datalogger control of reactor Prior to these experiments, the reactors had been fed either temperature, via 120–V, 50–W heating pads, surrounded by dairy or swine waste. At the start of these experiments, all fiberglass insulation to minimize heat loss. Gas production eight reactors were flushed to remove any residual solids. For from each reactor was monitored with submerged tipping the ASBR reactors, flushing consisted of transferring the buckets equipped with reed switches to enable electronic tip reactor contents to a holding tank, returning 10 L of sludge counting (Hawkins, 2000). The tipping buckets were cali- from the holding tank to the reactor, and then adding 15 L of brated to 30 mL tip–1, allowing measurement of biogas fresh wastewater to the reactor. For the DFAF reactors, production. flushing consisted of draining the reactor contents through Dairy wastewater ([COD] > 70 g L–1) was collected from the sludge drain port and then refilling with 25 L of fresh the liquid effluent side of a Vincent screw press (Model KP–6 wastewater. The reactors were then fed dilute dairy waste at w/2.3–mm screen, Vincent Corp., Tampa, Fla.) located at the an HRT of 0.5 d for six months. During this six–month period, Tennessee Agricultural Experiment Station Dairy near moderate COD removal was observed in most reactors, but Lewisburg, Tennessee. This concentrated material was gas production was far below the stoichiometrically pre- diluted to an approximate COD concentration of 5 g L–1. dicted 0.35 L (g COD)–1 (Speece, 1996). This suggested that Initially, this dilution was performed to facilitate reactor settling, not biodegradation, was the mechanism of COD start–up; the intention was to increase the influent concentra- removal, and motivated the sludge accumulation experi- tion significantly as the reactors came on line. During the ment. VFA removal experiments, the dilute manure solution was spiked with acetic acid to simulate the supernatant in an overloaded lagoon with an offensive odor (du Toit, 1987). SLUDGE ACCUMULATION EXPERIMENT Separate multi–channel peristaltic pumps (Model The following COD mass balance equation facilitated the 5773–80, Cole Palmer, Chicago, Ill.) were used to feed each sludge accumulation experiment: bank of four reactors. All pumps were controlled with Minf = Meff + Ms + MCH4 + Mbf + Mua (1) optically isolated relays (OAC5 or 120D10, Opto22, Teme- cula, Cal.) connected to the dataloggers. The DFAF reactors where were fed hourly, with the liquid level in the reactor being Minf = mass added by the influent controlled by the elevation of a standpipe and the hydraulic Meff = mass lost through the effluent retention time (HRT) being controlled through the volume of Ms = mass collected during reactor flushing liquid introduced to the reactor. MCH4 = estimated mass converted to methane The ASBR reactors were batch loaded every 6 h. The Mbf = mass from the backflush and draining ASBR react phase was 4 h, with mixing occurring for 5 min operation (DFAF reactors only) every h; the ASBR settling phase lasted 2 h. Reactor filling Mua = mass unaccounted for. occurred just prior to the react phase, and decanting took A one–month experiment was conducted to determine place immediately after the settling phase. A submersible values of the terms in equation 1 for each reactor. At the start pump (Model 1–42, Little Giant Pump Co., Oklahoma City, of the experiment, the ASBR reactors were drained and their Ok.) on each ASBR was used for effluent discharge and a sludge volume was set to 10 L; the DFAF reactors were separate pump (Model 1P372, Little Giant Pump Co.) mixed drained and refilled thrice with fresh wastewater to remove the reactor contents. The mixing pumps were sized to provide accumulated solids. The COD concentration of the sludge in vigorous agitation (0.4 – 0.6 kW m–3) of the reactor contents the ASBR reactors was measured to account for initial COD (Geankoplis, 1993). The HRT of an ASBR depends on the mass within those reactors. The reactors were then operated total fill–settle–react–decant time, as well as on the volume at a 0.5–d HRT for one month. During this time, influent and

Vol. 44(6): 1825–1831 1827 effluent volumes and COD concentrations were monitored removal of COD occurred in all reactors, the treatment daily to allow computation of Minf and Meff, and gas efficiencies were low (ca. 25% to 39%). Reactor 1 (R1) had production from each reactor was recorded to enable slightly better performance than did any of the others estimation of MCH4. (The MCH4 values were calculated reactors, but no other statistically significant performance assuming that 0.35 L of methane is produced per gram of differences were noted between reactors. The COD con- COD destroyed, and that the biogas composition was 75% centrations of the sludge after the one–month sludge methane.) After one month, each reactor was drained, and the accumulation test are presented in figure 5, with the feed volume and COD concentration of the drained liquid from COD indicated for purposes of comparison only. Reactor each reactor was measured to allow computation of Ms. The sludge concentrations were greatly enriched (ca. 5.8× to 10×) DFAF reactors were also backflushed and drained twice to compared to the feed, suggesting that significant solid effectively remove all accumulated solids; the volumes and settling had occurred. COD concentrations of each backflush and drain were The gas collection system for the ASBR reactors malfunc- measured to allow computation of Mbf. tioned during the experiment, thus no gas data were collected for these reactors. However, previous experience suggested VOLATILE FATTY ACIDS REMOVAL EXPERIMENT that the biogas production in the ASBR reactors was The results of the sludge accumulation experiment negligible; it is therefore reported as zero in table 2. In the strongly suggested that negligible biodegradation was occur- DFAF reactors, MCH4 values were greater at 35°C than at ring at the 0.5–d HRT. Although other researchers have 25°C, probably reflecting greater hydrolysis and subsequent shown that longer retention time reactors can treat dairy fermentation and methanogenesis at the higher temperature. waste (Viraraghavan and Kikkeri, 1990; Monroy et al., 1994; The results of the sludge accumulation experiment are Dugba and Zhang, 1999), the project goal remained explor- summarized in table 2. Overall, the mass balance appeared ing the viability of short retention time reactors for reducing tight, with less than 7% of the mass unaccounted for in any and preventing VFA accumulation in overloaded lagoons. reactor (table 2, Mua/Minf column). Settling accounted for Therefore, the decision was made to explore the performance 76% to 121% of the mass removal in the ASBR reactors, with of the reactors to remove VFAs from VFA–enriched dairy no apparent correlation between reactor temperature and wastewaters, such as might occur in overloaded lagoons. In settled mass. It is counterintuitive for the settled mass to this model (diagrammed in fig. 1, configuration B), the exceed the mass removed from the wastewater, but there are high–rate anaerobic digesters treat supernatant from the lagoon rather than the screened raw wastewater. In so doing, 6 the lagoon serves as a settling tank and as a hydrolysis and )

5 ÓÓ fermentation reactor, while the primary function of the –1 ASBR DFAF high–rate digester is to support a large methanogenic 4 ÓÓ

population capable of converting VFAs to biogas. The

ÓÓ ÖÖÒÕÕŠÚÚÓÛÛ

critical parameter in such a configuration is the maximum 3

ÓÓ Ô ÒÕÕŠÚÚÓÛÛ volumetric VFA removal rate achievable in the high–rate ÖÖ

ÓÓ Ô ÒÕÕŠÚÚÓÛÛ

anaerobic digesters; knowledge of this parameter would ÖÖ

facilitate a preliminary economic analysis. To estimate this

ÓÓ Ô ÒÕÕŠÚÚÓÛÛ –1 1 ÖÖ

parameter, the reactors were fed dilute (5 g COD L ) dairy COD Concentration (g L

ÓÓ Ô ÒÕÕŠÚÚÓÛÛ waste spiked with acetic acid at concentrations of 0.5, 1.5, ÖÖ 0

and 3.0 g HAc L–1, and operated at a 0.5–d HRT. Additional-

ÓÓ Ô ÒÕÕŠÚÚÓÛÛ Feed R1 ÖÖR3 R2 R4 R5 R7 R6 R8 ly, the DFAF reactors were operated at HRTs of 0.25 and (255C) (255C) (355C) (355C) (255C) (255C) (355C) (355C) 0.125 d at a concentration of 3 g HAc L–1. After each change Reactor Number in influent VFA concentration, reactors were operated for at Figure 4. Average chemical oxygen demand (COD) concentration of in- least three hydraulic retention times prior to the beginning of fluent (feed) and effluent for the sludge accumulation test. data collection. 60

ASBR DFAF Õ

) 50

RESULTS AND DISCUSSION –1

ÚÚÕ Ö

SLUDGE ACCUMULATION EXPERIMENT 40

ÔÔŠÚÚÕÛÛÖ The working volume of the ASBR reactors remained at Ó

ÒÒ ÔÔŠÚÚÕÛÛÖ 25 L due to the absence of any fill material. However, during Ó

the first six months of operation, the working volumes of the

ÒÒ ÔÔŠÚÚÕÛÛÖ

DFAF reactors dropped significantly due to biomass growth 20 Ó

ÒÒ ÔÔŠÚÚÕÛÛÖ on the packing material. This growth was quantified by 10 Ó

measuring the refill volume after flushing: the working COD Concentration (g L

ÚÚ ÒÒ ÓÔÔŠÚÚÕÛÛÖ

volumes of reactors 5, 7, and 8 had dropped from 25 to 15 L, 0

ÚÚ ÒÒ ÔÔŠÚÚÕÛÛÖ and reactor 6 had dropped to 11.5 L. Feed *Ó R1 R3 R2 R4 R5 R7 R6 R8 The average COD concentrations of the influent and (255C) (255C) (355C) (355C) (255C) (255C) (355C) (355C) effluent during the sludge accumulation experiment are Reactor Number presented in figure 4. The average influent concentration of Figure 5. Chemical oxygen demand (COD) of initial sludge drain from –1 –1 reactors after one–month duration sludge accumulation test. *Feed con- the wastewater was 5.2 g L (±0.9 g L ), yielding Minf of 2160 g for all eight reactors. While a statistically significant centration is shown for purposes of comparison.

1828 TRANSACTIONS OF THE ASAE Table 2. Results from the sludge accumulation test. Minf = chemical oxygen demand (COD) added by the influent, Meff = mass lost through the effluent, Ms = mass collected during reactor flushing, MCH4 = estimated mass converted to methane, Mbf = mass from the backflush and draining of the DFAF reactors only, and Mua = mass unaccounted for. Rightmost column indicates the ratio of COD settled to COD removed; the uncertainty in the rightmost column was estimated as +16%. Reactor number, type, and Minf Meff Ms MCH4 Mbf Mua/Minf Ms/(Minf – Meff) operating temperature (°C) (g) (g) (g) (g) (g) (%) (%) 1 ASBR 25 2160 1310 787 0.0 0.0 2.8 93 2 ASBR 35 2160 1570 448 0.0 0.0 6.7 76 3 ASBR 25 2160 1640 629 0.0 0.0 –5.0 121[a] 4 ASBR 35 2160 1530 691 0.0 0.0 –2.9 110[a] 5 DFAF 25 2160 1560 519 72.8 62.5 –2.8 87 6 DFAF 35 2160 1580 335 135.0 57.1 2.5 58 7 DFAF 25 2160 1540 549 33.0 55.6 –0.9 89 8 DFAF 35 2160 1540 347 97.4 58.4 5.5 56 [a] Values above 100% are not physically reasonable and are due to measurement errors. two factors that could account for this: error in the (P = 0.084), while the other ASBR reactors achieved only measurement of Ms, or significant temporal variations in slightly significant treatment (P < 0.025). The low removal influent and effluent COD concentration. The former appears observed in R1 at all concentrations could not be explained. more likely, since the particulate nature of the ASBR sludge At an influent VFA concentration of 0.5 g HAc L–1, VFA made accurate COD measurements challenging. reduction was slightly greater (though not to a statistically A propagation of error calculation, based on estimated significant degree) in the ASBR reactors than in the DFAF uncertainties in the raw volume and COD concentration reactors. Under these conditions, all reactors except R1 and measurements, suggested that an uncertainty of ±16% would R7 achieved VFA removals greater than 50%. However, be likely in the Ms/(Minf – Meff) values. (Significant temporal there was no statistically significant difference in the effluent variations in COD concentration appear a less likely source concentrations between reactor types at 25°C (P = 0.65) or at of error, since the low values of Mua/Minf suggest that the mass 35°C (P = 0.34). balance was generally accurate.) Settling accounted for 56% At an influent VFA concentration of 1.5 g HAc L–1, VFA to 89% of the mass removal in the DFAF reactors, with treatment efficiency in all the ASBR reactors dropped below reactor temperature significantly impacting the fraction of 51%. This decrease suggested that the ASBR reactors were COD settled. Specifically, settling accounted for 88% of the reaching their maximum removal potential. The DFAF removal in the 25°C reactors, but only 57% of the removal in reactors at the same 1.5 g HAc L–1 feed concentration the 35°C reactors. The greater MCH4 values and lower solids achieved greater than 65% removal, except R7, which accumulations in the 35°C DFAF reactors strongly support achieved 45%. Interestingly, three DFAF reactors achieved the hypothesis that hydrolysis limited the degradation of greater VFA treatment efficiencies at the increased loading dairy wastewater at the 0.5–d HRT. rate, perhaps reflecting recruitment of more organisms in the reactor. Reactor type significantly affected treatment effi- VFA REMOVAL EXPERIMENT ciency at this VFA concentration, with DFAF reactors The VFA treatment efficiencies during the steady–state outperforming the ASBR systems at both 25°C (P = 0.017) period for each reactor and feed concentration are presented and 35°C (P < 0.0001). in figure 6. All reactors achieved statistically significant VFA At 3.0 g HAc L–1, no ASBR reactors attained treatment removal at influent VFA concentrations of 0.5 and 1.5 g HAc efficiencies greater than 20%. In contrast, DFAF reactors, L–1 (P < 0.0001). However, at an influent concentration of with the exception of R7, had treatment efficiencies greater 3.0 g HAc L–1, R1 did not achieve significant VFA reduction than 80%. As with the 1.5 g HAc L–1 feedstock, there was a highly statistically significant difference in performance 0.5 g HAc L 1.5 g HAc L 3.0 g HAc L between reactor types at both at both 25°C and 35°C (P < 120 0.0001).

100 With treatment efficiencies of greater than 80% for the

DFAF reactors, the HRT was dropped to 0.25 d and 0.125 d

80 while maintaining the VFA concentration at 3.0 g HAc L–1.

Ž Figure 7 summarizes the results, with the leftmost bar in each

Ž set being a reproduction of the rightmost bar from the DFAF –3 –1

40 data in figure 6. At a loading of 12 kg HAc m d (i.e., 3.0 g

Ž HAc L–1 at 0.25 d) R5, R6, and R8 had treatment efficiencies

Ž greater than 75%, while R7 fell to slightly below 57%. A

VFA Treatment Efficiency (%) Treatment VFA comparison of the replicates showed a significant difference

ÜÜÙ ŸŸŸ 0 Ž R1 R3 R2 R4 R5 R7 R6 R8 between the 25°C reactors (R5 and R7) (P < 0.0001), but no (255C) (255C) (355C) (355C) (255C) (255C) (355C) (355C) ° ASBR ASBR ASBR ASBR DFAF DFAF DFAF DFAF significant difference was seen between the 35 C reactors Reactor Number (R6 and R8) (P = 0.52). The difference between R5 and R7 Figure 6. Volatile fatty acid (VFA) treatment efficiencies for reactors op- is significant; however, when the replicate averages are erating at 0.5–d hydraulic retention times, and at three VFA inlet con- compared across temperatures, there is no significant centrations (expressed as HAc, acetic acid). Reactor numbers are difference (P < 0.0001). non–sequential to keep replicate reactors adjacent to each other.

Vol. 44(6): 1825–1831 1829

0.5–d HRT 0.25–d HRT 0.125–d HRT recommended lagoon loading rate can be expressed as 100 0.072 kg COD m–3 d–1. One unit of acetic acid exerts 90 1.07 units of COD. Therefore, the volumetric COD removal –3 –1 80 rate in the DFAF reactors can be estimated as 9.0 kg m d , × 70 or 125 the suggested loading rate for a lagoon in Tennessee. To achieve VFA removal rates of 125× that of a lagoon, the 60 VFA concentration entering the DFAF must be rather high

50 (3 g HAc L–1), implying that the lagoon supernatant would

Ÿ ÙÙ

40 have this VFA concentration. While the DFAF could

Ÿ ÙÙ

30 conceivably be employed to “rescue” an overloaded lagoon

Ÿ ÙÙ that had reached such high VFA concentrations, one would

20 not intentionally design a system to operate in this range,

Ÿ ÙÙ

VFA Treatment Efficiency (%) Treatment VFA 10 since such a high VFA concentration in the lagoon would

Ÿ ÙÙ 0 produce highly offensive odors. Instead, to avoid significant R5 (255C) R7 (255C) R6 (355C) R8 (355C) odor problems, a lagoon/DFAF system might be expected to Reactor Numbers operate at VFA concentrations of 0.3 g HAc L–1 (du Toit, Figure 7. Volatile fatty acid (VFA) removal efficiencies for the downflow 1987). In this case, a DFAF operating at a 0.125 d HRT would anaerobic filter (DFAF) reactors receiving acetic acid at 3.0 g L–1 and op- receive VFA at 2.4 kg HAc m–3 d–1. Under these conditions, erated at hydraulic retention times (HRT) of 0.5, 0.25, and 0.125 d. Reac- tor numbers are non–sequential to keep replicate reactors adjacent to the data presented in figure 8 suggest that a removal rate of –3 –1 each other. 1.7 kg HAc m d would be achievable. Comparing this rate to the published lagoon removal rates (Safley and Wester- At the highest VFA loading rate of 24 kg HAc m–3 d–1 man, 1992) suggests that the DFAF could remove VFA at 25× (i.e., 3.0 g HAc L–1 at 0.125 d) the VFA treatment efficiencies that achieved by anaerobic lagoons in Tennessee. ranged from 31% to 52%. At this loading rate, a comparison of the reactors at the two temperatures shows a significant COMPARISON OF ASBR AND DFAF REACTORS difference (P < 0.0001) between the effluent VFA concentra- A comparison of the reactors at a loading of 1 kg HAc m–3 tions. Likewise, there is a significant difference between the d–1 shows no significant difference (P = 0.534 at 25°C, and replicate reactors (P < 0.0007 for both temperatures). P = 0.336 at 35°C). Likewise, there is no difference between A plot of the VFA loading rate and VFA removal rate data the reactors when operated at 3 kg HAc m–3 d–1 at either collected from the VFA experiment is shown in figure 8. The temperature (P = 0.0001). As the loading increased to 6 kg data was fit to a hyperbolic parabola (e.g., Monod or HAc m–3 d–1, it became apparent that the ASBR reactors had Michaelis–Menten equation) with r2 values of 0.85 and 0.90 reached a point that they could not effectively treat the for the 25°C and 35°C data, respectively. The maximum VFA wastewater; however, treatment efficiency of the DFAF removal rates predicted by the curve were 15 kg HAc m–3 d–1 reactors continued to increase. As expected, there was a at 25°C and 20 kg HAc m–3 d–1 at 35°C. The half saturation difference between the two reactor types at both temperatures constants were likewise determined to be 16.5 and 18.5 kg (P = 0.0001). This large difference in the treatment efficien- HAc m–3 d–1 for 25°C and 35°C, respectively. cies for the two reactors presents good evidence that the The 70% VFA treatment efficiency observed in DFAF DFAF reactor is better suited for removal of VFAs at these reactors receiving VFA loadings of 12.0 kg HAc m–3 d–1 higher loading rates. translates into a volumetric VFA removal rate of 8.4 kg HAc On the surface, the twenty–five–fold performance differ- m–3 d–1. To put this number into perspective, it can be ence between a DFAF and a lagoon would appear to support compared to the recommended maximum volatile solids the feasibility of the approach proposed herein. However, (VS) loading rates for lagoons in Tennessee, which ASAE there are large differences between the volumetric costs of gives as 0.072 kg VS m–3 d–1 (ASAE Standards, 1999). Since lagoons (simple earthen structures) and DFAF reactors the VS to COD ratio for dairy wastes is close to unity, the (typically more complex concrete or steel structures). Furthermore, this experiment was conducted with the use of a synthetic lagoon supernatant. An economic analysis based 12 on the reactor performance presented here should be

10 conducted. If the approach appears economically feasible, ) then the next experiment should involve pilot–scale testing –1

d 8 using actual lagoon supernatant. –3

6 DFAF @ 25 C CONCLUSION 4 DFAF @ 35 C The removal of organic matter in anaerobic sequencing Fit to 25 C Fit to 35 C batch reactors (ASBR) and downflow anaerobic filter 2 (DFAF) reactors operating at 0.5–d hydraulic retention time was shown to occur mainly through solids settling in the

VFA Removal Rate (kg m VFA 0 0 5 10 15 20 25 reactors. The chemical oxygen demand (COD) mass balance VFA Loading Rate (kg –3m d–1) on the ASBR reactors showed settling could account for all of the COD removal observed. In contrast, settling accounted Figure 8. VFA removal kinetics of the DFAF reactors. for 55% to 88% of the observed COD removal in the DFAF

1830 TRANSACTIONS OF THE ASAE reactors. Furthermore, the differences between the accumu- Geankoplis, C. J. 1993. Transport Processes and Unit Operations. lated COD in the 25°C and 35°C DFAF reactors supported the 3rd ed. R. P. Rottino, ed. Englewood Cliffs, N.J.: Prentice–Hall, idea that hydrolysis limits the biodegradation of dairy Inc. wastewater at very short retention times. Hawkins, G. L. 2000. Reducing organic loading of anaerobic At low loading rates (<3.0 kg HAc m–3 d–1), both the lagoons with high–rate anaerobic digestion: Reactor performance and economic analysis. PhD diss. Knoxville, Tenn.: ASBR and DFAF reactors achieved moderate volatile fatty University of Tennessee, –3 –1 acid (VFA) removal. At a loading rate of 6.0 kg HAc m d , Hawkins, G. L., and D. R. Raman. 1999. Kinetic study of anaerobic the ASBR reactors were ineffective, while the DFAF reactors filter and anaerobic sequencing batch reactor for design of a efficiently removed more than 80% of the applied acetic acid. pretreatment system for dairy and swine wastewater. ASAE As loading rates increased to 24 kg HAc m–3 d–1, the Paper No. 99–4061. St. Joseph, Mich.: ASAE. treatment efficiency of the DFAFs dropped off gradually to Hernandez, E. P. S., and X. Rodriguez. 1992. Treatment of settled 50%. Based on these experiments, the DFAF reactors appear cattle–wastewaters by downflow anaerobic filter. Short well suited to the task of rapidly removing accumulated VFA Communication. Bioresource Tech. 40(1): 77–79. from overloaded lagoons. A thorough economic analysis is Hobbs, P., B. F. Pain, and T. H. Misselbrook. 1995. Odorous needed to determine the feasibility of such an endeavor. compounds and their emission rates from livestock waste. In New Knowledge in Livestock Odor Solutions: Proc. International Livestock Odor Conf. ’95, 11–15. Ames, Iowa: ACKNOWLEDGEMENTS Iowa State University. The authors would like to extend their sincere appreci- Monroy, O., K. A. Johnson, A. D. Wheatley, F. Hawkes, and M. ation to Paul Elliott, Craig Wagoner, Walker Garner, Caine. 1994. The anaerobic filtration of dairy waste: Results of a Alexandra Pinto, and Lara Moody. Financial support for this pilot trial. Bioresource Tech. 50(3): 243–251. work was provided by the United States Geological Survey Powers, W. J., H. H. Van Horn, A. C. Wilkie, C. J. Wilcox, and R. Tennessee State Water Resources Research Institute Program A. Nordstedt. 1999. Effects of anaerobic digestion and additives Grant 99–4370–18500, by the Tennessee Agricultural Exper- to effluent or cattle feed on odor and odorant concentrations. J. iment Station Regional Project S–275, and by the Raymond Animal Science 77(6): 1412–1421. Powers, W. J., and H. H. Van Horn. 1998. Dietary manipulation to M. Cash Foundation. alter manure composition and odor. In Proc. Animal Production Systems and the Environment: An International Conference on Odor, Water Quality, Nutrient Management, and Socioeconomic REFERENCES Issues, 91–96. Ames, Iowa: Iowa State University. Safley, L. M., and P. W. Westerman. 1992. Performance of a dairy APHA. 1998. Standard Methods for Examination of Water and manure anaerobic lagoon. Bioresource Tech. 42(1): 43–52. Wastewater. 20th ed. Washington, D.C.: American Public Health SAS. 1999. SAS version 8.0. Cary, N.C.: SAS Institute, Inc. Association. Speece, R. E. 1996. Anaerobic Biotechnology for Industrial Arceivala, S. J. 1981. Disposal of waste on land. In Wastewater Wastewaters. Nashville, Tenn.: Archae Press. Treatment and Disposal: Engineering and Ecology in Pollution Sung, S., and R. R. Dague. 1992. Fundamental principles of the Control, 429–527. P. N. Cheremisinoff, ed. New York, N.Y.: anaerobic sequencing batch reactor process. In Proc. 47th Marcel Dekker, Inc. Purdue Industrial Waste Conf., 393–408. Chelsea, Mich.: Lewis ASAE Standards, 46th ed. 1999. ANSI/ASAE EP403.3. Design of Publishers, Inc. anaerobic lagoons for animal waste management. St. Joseph, USDA–SCS. 1992. National Field Engineering Handbook: Part Mich.: ASAE. 651. Agricultural Waste Management Field Handbook Colorado Aggregate Company. 1988. MSDS for Landscape Gravel, (AWMFH). Publication No. 210–VI–AWMFH. Aquarium Gravel, and Barbecue Briquettes. Alamosa, Colo.: Viraraghavan, T., and S. R. Kikkeri. 1990. Effect of temperature on Mountain West – Colorado Aggregate Company. anaerobic filter treatment of dairy wastewater. Water Sci. Tech. du Toit, A. J. 1987. Quantification of odour problems associated 22(9): 191–198. with liquid and solid feedlot and poultry wastes. Water Pollution Wilkie, A., and E. Colleran. 1989. The development of the Research and Control 19: 31–41. anaerobic fixed–bed reactor and its application to the treatment Dugba, P. N., and R. Zhang. 1999. Treatment of dairy wastewater of agricultural and industrial wastes. In International with two–stage anaerobic sequencing batch reactor systems: BiosystemsVolume III, 183–226. D. L. Wise, ed. Boca Raton, Thermophilic versus mesophilic operations. Bioresource Tech. Fla.: CRC, Inc. 68(3): 225–233.

Vol. 44(6): 1825–1831 1831 1832 TRANSACTIONS OF THE ASAE