Upflow biological aerated filters for the treatment of flushed swine manure

Bioresource Technology 74 (2000) 181±190

Up¯ow biological aerated ®lters for the treatment of ¯ushed swine manure P.W. Westerman a,*, J.R. Bicudo b, A. Kantardjie€ c a

Department of Biological and Agricultural Engineering, North Carolina State University, Campus Box 7625, Raleigh, NC 27695-7625, USA b Department of Biosystems and Agricultural Engineering, University of Minnesota, St. Paul, MN 55108-6005, USA c Ekokan, Inc., P.O. Box 5354, Cary, NC 27512, USA Received 26 April 1999; received in revised form 23 December 1999; accepted 6 February 2000

Abstract A pilot plant with capacity to treat up to 8 m3 /day of supernate from settled ¯ushed swine wastes was monitored for 12 months. The main system is composed of two up¯ow aerated bio®lters connected in series. The aerated bio®lters, operated under warm weather conditions (average temperature of 27°C), were able to remove about 88% of biochemical oxygen demand (BOD), 75% of chemical oxygen demand (COD), and 82% of total suspended solids (SS) with loading of 5.7 kg COD/m3 /day of bio®lter media. The total Kjeldahl nitrogen (TKN), total ammonia nitrogen (NH3 -N), and total nitrogen (Total-N) reductions averaged 84%, 94% and 61%, respectively, during warm weather, with a signi®cant portion of the NH3 -N being converted to nitrite plus nitrate nitrogen (NO2 ‡ NO3 -N). At higher organic loading (over 9 kg COD/m3 /day) during September, the bio®lters had only slightly lower percentage removal rates. Operation at lower temperatures (average of 10°C) resulted in lower performances. The COD, TKN, NH3 -N, and Total-N removal averaged 56%, 49%, 52%, and 29%, respectively, in December through March. The COD mass removal rate was linear with loading rate over the range of approximately 2±12 kg COD/m3 /day of ®lter. A mass balance average for the 12 months indicated that about 30% of the in¯uent volume, 35% of Total-N and 60% of total phosphorus (Total-P) are removed with the bio®lter backwash. Management and utilization of the backwash are important factors in implementing this type of system on farms. The unaccounted-for nitrogen was about 24% and could have been lost as ammonia volatilization or possibly through denitri®cation within the bio®lm. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Flushed swine manure; Biological aerated ®lter; Bio®lter; Nitri®cation

1. Introduction Over the past 30 years, rearing of swine has shifted from small, outdoor herds to larger, more specialized con®nement facilities. Raising food animals in North Carolina, particularly poultry and pigs, has become the most valuable segment of agriculture. Swine are worth close to $2 billion annually in North Carolina (NCSU, 1997). With rising environmental concerns, increased media attention and tighter governmental regulations, managing animal wastes in an environmentally responsible and economically feasible way can be a challenge. With goals to reduce odor and ammonia volatilization from swine farms, aerobic treatment systems for swine wastes are being evaluated. *

Corresponding author. Tel.: +1-919-515-6742; fax: +1-919-5157760. E-mail address: [email protected] (P.W. Westerman).

Burton (1992) reviewed strategies for the aerobic treatment of pig slurry to meet objectives such as odor control, nitrogen management, carbon removal, pathogen control and heat recovery. More recently, Westerman and Zhang (1997) reviewed aeration of livestock manure and lagoon liquid for odor control. These and other reviews indicate a number of potential bene®ts of aerobic treatment, including odor abatement, removal of surplus nitrogen, and improved handling in storage and during spreading. However, the cost and level of management skills required to install and operate aerobic treatment systems must also be considered when evaluating implementation of such systems on farms. Aerobic treatment of swine manure has been studied for over 20 years using various systems. Research e€orts have focused mainly on suspended growth processes. Experimental studies have been conducted not only under controlled laboratory conditions (e.g., Owens et al., 1973; Smith and Evans, 1982; Evans et al., 1986;

0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 2 8 - 6

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Oleszkiewicz, 1986; Osada et al., 1991; Bortone et al., 1992), but also at both pilot and full scale (e.g., Fallow®eld et al., 1992; Liao and Maekawa, 1994; Bicudo and Svoboda, 1995; Burton and Sneath, 1995; Yang et al., 1997). For municipal wastewater treatment, there are fewer full-scale applications of attached growth systems in the US compared to suspended growth applications, especially for biological nutrient removal and nitri®cation (WERF, 1994). Packed bed reactors and biological aerated ®lters (BAF) represent attached growth processes that have been utilized to some extent for nitri®cation of municipal and industrial wastewaters. Unlike trickling ®lters, the hydraulic design of these systems is such that the media are submerged in the reactor liquid. The media are stationary during normal operation, held in place by gravity. Aeration in a BAF is provided through an air di€usion system located at the bottom of the ®lter. The ®rst development of a packed bed reactor for nitri®cation that met with widespread commercial success was the BAF (Sammut et al., 1994). Initial BAF applications were for carbon oxidation but were rapidly extended ®rst to combined carbon oxidation/nitri®cation and subsequently to separate stage applications (Toettrup et al., 1994). In the typical operation of a BAF, the media is periodically backwashed with air scours and liquid ¯ushes to release total suspended solids (SS) trapped in the voids of the packed bed and to control the extent of ®lm growth on the media surface. The primary advantage of the BAF is biological treatment and solidsÕ separation in the same reactor eliminating the requirement for separate secondary clari®cation. As a consequence, the technology could reduce the space requirements for treatment relative to more conventional technologies such as activated sludge systems. The BAF have been applied to nitrogen removal using external carbon sources and simultaneous biological phosphorus and nitrogen removal using a wastewater carbon source (Sammut et al., 1994). Several commercial technologies originally developed in Europe, were ®rst introduced into the North American (US and Canada) market in the late 1980s through demonstration projects (Stensel et al., 1988). As pointed out in WERF (1994), the technology would bene®t from demonstration applications in which the robustness of the technology and ease of operation could be tested under typical operating conditions. In recent years, BAF has been successfully applied to pulp and paper mill e‚uents (Kantardjie€ and Jones, 1997) and slaughterhouse e‚uent (Kantardjie€ and Jones, 1996). The performance of packed bed reactors for the treatment of swine manure has been described by Boiran et al. (1996). They conducted a study to test the operation of a laboratory bio®lm in®ltration±percolation aerated system (BIPAS) reactor for the treatment of piggery wastes from a lagoon with three monthsÕ storage

time. The reactors (with bed volume of about 9 l) were ®lled with calcareous or siliceous gravel as support materials. Aeration was intermittent, varying from 5 min every 30 min to 5 min every 6 h, and lagoon liquid was fed to the top of the reactors intermittently for about 10 min every hour. Each reactor treated 0.5 l/day of lagoon liquid. Average in¯uent characteristics were: chemical oxygen demand (COD) of 3200 mg/l, total solids (TS) of 0.36%, total Kjeldahl nitrogen (TKN) of 1090 mg/l and total phosphorus (Total-P) of 260 mg/l. Results with calcareous gravel and a 5-min ``on'' ± 25 min. ``o€'' intermittent aeration showed COD removal of 70%, complete removal of ammonia, and 25±38% removal of total nitrogen (Total-N). Current animal waste research at North Carolina State University includes evaluation of innovative treatment technologies for swine wastes. One of the technologies evaluated consisted of two up¯ow aerated bio®lters connected in series for the treatment of ¯ushed swine manure after solids settlement. The main objective was to evaluate the high-rate aerated bio®lter system for treatment of separated liquid obtained after solids separation (in concrete settling basins) in terms of organic matter (biochemical oxygen demand, BOD, and COD), solids, nutrients (N and P) and odor removal. The experimental methodology and results obtained from a one-year evaluation period of this technology are presented in the study. 2. Methods 2.1. System description and operation A pilot plant with capacity to treat up to 8 m3 /day was installed in May 1997 at the NCSU Lake Wheeler Road Field Laboratory, Swine Educational Unit. A ¯ow diagram for the system is presented in Fig. 1. The main system was composed of two up¯ow bio®lters connected in series, three blowers and a polishing tank. The bio®lter columns each had a cross-sectional area of 0.5 m2 and height of 3.5 m (volume of 1.75 m3 ). They were packed to a 3-m height with plastic media (1.5 m3 of media in each bio®lter) with a speci®c surface area of about 140 m2 /m3 . Liquid volume was reduced to 1.2 m3 for each bio®lter with media present. The treatment system was monitored continuously for one year, starting in June 1997. Flushed wastes (each building ¯ushed four times a day) from a swine research unit with population of about 325 sows, 25 boars, 350 nursery pigs and 500 ®nishing pigs were collected into a 26.5 m3 concrete settling basin. A small portion of the over¯ow from settling basin was pumped into a 4.2-m3 storage tank. Waste was pumped from the storage tank to the ®rst bio®lter and traveled upward inside the bio®lter. The

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183

Fig. 1. Flow diagram of the aerobic bio®lters pilot plant.

partially treated e‚uent exited into a pipe at the top, and gravity fed the second bio®lter. The treated e‚uent from the second bio®lter exited by pipe at the top and was allowed to settle in the polishing tank. The clari®ed e‚uent was discharged to an anaerobic lagoon, which was part of the farmÕs existing treatment system. The bio®lters were aerated using three blowers, one for each bio®lter and a third for air scouring during backwash (Aerzen GM 3S, Aerzen, Canada) with capacity of about 1.2 m3 /min at 1900 rpm and 40 kPa pressure. The capacity of the blowers exceeded the aeration requirements, so approximately 0.2 m3 /min ¯owed to each bio®lter and about 1.0 m3 /min was bypassed (discharged directly into the atmosphere). Air was di€used through tube di€users located at the bottom of the bio®lters. A design criterion was to supply air at 25 m3 of air per meter square of bio®lter cross-sectional area per hour. Dissolved oxygen (DO) was normally between 6 and 8 mg/l in e‚uent from each bio®lter, but ranged from about 4 to 11 mg/l. The bio®lters were backwashed periodically by agitating with increased air ¯ow so that the accumulated suspended solids and newly produced biomass were removed from the system. Typically, the backwash frequency was 4 times a day for the ®rst bio®lter and once in every two days for the second bio®lter. For the backwash cycle, approximately 25 cm of liquid depth was removed from the bio®lter (out the bottom) to prevent over¯ow during the increased air¯ow. This was followed by diversion of air from all three blowers to the bio®lter for 3 min. The more concentrated liquid was removed from the bottom of the bio®lter. Nearly 30% of bio®lter water volume was removed during backwash

and stored in a tank where solids settled as biosolids. The upper liquid portion of the backwash was discharged back to the settling basin. Loading rate of the bio®lters was based on COD and was approximately 6 kg COD/m3 /day to the ®rst bio®lter, except during September 1997 when a higher loading rate of about 9.6 kg/m3 /day was used to test limits of the bio®lter. The hydraulic ¯ow rates to the ®rst bio®lter were normally 4±5 m3 /day, except September 1997 when ¯ow rate was about approximately 8 m3 /day. 2.2. Sampling and analyses Samples were collected weekly from June 1997 to October 1997. From November 1997 onwards they were collected biweekly. Grab samples were collected for in¯uent and e‚uent of Bio®lter #1 (BIO1-IN and BIO1OUT, respectively), e‚uent from Bio®lter #2 (BIO2OUT), and backwash samples from both bio®lters (BIO1-BW and BIO2-BW; Fig. 1). Samples were analyzed for TS, volatile solids (VS), SS, COD, BOD, TKN, total ammonia nitrogen (NH3 -N), nitrite nitrogen plus nitrate nitrogen (NO2 -N+NO3 -N), Total-P and orthophosphate phosphorus (OPO4 -P). Chemical and biochemical analyses were performed according to USEPA (1979), standard methods (Clesceri et al., 1990) or TTIM (1973) with slight modi®cations including dialysis of samples for nutrient analysis. Air and water temperatures, pH and DO were routinely monitored during the evaluation period on a daily basis. Flow rates to the bio®lter were measured with a magnetic ¯ow meter (Promag 30F, Endress & Hauser).

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Liquid samples were also taken for odor evaluation on four di€erent occasions. Duplicate samples were collected on 4 August 1997, 21 October 1997, 9 February 1998 and 23 March 1998 from the following sampling points: SBW-IN (¯ushed wastewater entering the settling basin), BIO1-IN, BIO1-OUT, BIO2-OUT, DEN1-OUT (e‚uent from the polishing tank), BIO1BW (except 4 August 1997) and BIO2-BW (except 4 August 1997; Fig. 1). They were analyzed for odor intensity (concentration), odor irritation intensity and odor quality (pleasantness or unpleasantness). Descriptive scales for odor intensity, irritation intensity and pleasantness utilized by odor panelists are summarized in Table 1. A trained odor panel at Duke University evaluated all samples. Positive control (butyric acid) and blank samples (fabric and water, not exposed to odorants) were used throughout the evaluation. 2.3. Statistical analyses For liquid and biosolids data, 95% con®dence intervals for means and linear regression of data were calculated using Excel (MicrosoftÒ). Di€erences in means were tested at the P < 0:05 level. Comparison of two data sets was made by subtracting one data set from another and determining the 95% con®dence interval for the di€erence. Means were considered signi®cantly different at P < 0:05 if the 95% con®dence interval did not include 0. If the con®dence level is 0.05, then the con®dence interval is given by   r x  1:96 p ; n where x is the mean of the sample (in this case, the mean of the di€erence between two data sets), r the standard deviation and n is the number of observations. The 1.96 is the value from the standard normal distribution corresponding to 95% con®dence. For odor data, the SAS general linear model procedure was used to test signi®Table 1 Descriptive scales for odor intensity, irritation intensity and pleasantness Odor intensity and irritation intensity

Pleasantness

Description

Scale

Description

Scale

Maximal Very strong Strong Moderate strong Moderate Moderate weak Weak Very weak None at all

8 7 6 5 4 3 2 1 0

Extremely unpleasant Very unpleasant Moderately unpleasant Slightly unpleasant Neutral Slightly pleasant Moderately pleasant Very pleasant Extremely pleasant

8 7 6 5 4 3 2 1 0

cance (P < 0:05) of di€erence in means using the leastsquare means procedure (SAS Institute, 1996). 3. Results and discussion 3.1. Overall reductions in bio®lters The in¯uent concentrations to each of the two bio®lters connected in series and the reductions in each bio®lter are shown in Table 2 averaged for one year (June 1997±May 1998). For most parameters, the concentration reduction in BIO1 was greater than in BIO2. On a percentage basis, some of the reductions were similar for BIO1 and BIO2. In principle, heterotrophic bacteria, which would reduce COD and convert organic nitrogen to ammonium nitrogen, should dominate in BIO1, and BIO2 should be dominated by nitrifying bacteria (Nitrosomonas and Nitrobacter). The biological activity in both bio®lters should be a€ected by temperature, but the nitrifying bacteria may be more sensitive to temperature and pH (Hargrove et al., 1996). As is discussed in Section 3.2, the nitri®cation in BIO1 was reduced with lower temperature, and thus most of the nitri®cation during low temperatures was in BIO2. The COD reduction seemed less a€ected by temperature, as is discussed in Section 3.4. The relative reductions are similar for COD, VS and SS. Overall reductions for COD, VS and SS were 72%, 57% and 76%, respectively (Table 2). Some of the SS may be ®ltered by the media in the bio®lter, but the SS and VS are also being utilized by the bacteria and converted to microbial biomass. A signi®cant portion of the biomass is removed with the periodic backwash of the bio®lters. Nitrogen reductions shown in Table 2 indicate that NH3 -N is reduced and NO2 ‡ NO3 -N is increased through the bio®lters. The NH3 -N decreased by 82%. The concentration of nitrite plus nitrate nitrogen NO2 ‡ NO3 -N in the e‚uent was about 60 mg/l while Total-N was about 140 mg/l. There was an overall reduction in TKN and Total-N (72% and 49%, respectively; Table 2). A portion of this nitrogen was removed with backwash, but the mass balance would also show overall loss of nitrogen, which would logically be assumed to be ammonia volatilization or through simultaneous nitri®cation/denitri®cation in the bio®lm. Although neither volatilization nor denitri®cation were directly measured or calculated individually for this study, there is evidence from this and other experimental studies that simultaneous nitri®cation/denitri®cation may have occurred within the bio®lm (e.g., Laursen et al., 1994). The denitri®cation process occurs in anoxic conditions when there is enough organic matter to be used as an electron donor. Denitri®cation processes in BAF are

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Table 2 Average bio®lter performance under an average loading of 6.6 kg COD/m3 /day and temperature of 24.2°C (June 1997±May 1998) Parameter

In¯uent (mg/l)

Bio®lter 1

Bio®lter 2

% Red.

mg/l

% Red.

mg/l

COD TS VS SS TKN NH3 -N NO2 ‡ NO3 -N Total-N Total-P OPO4 -P

1842 2346 1266 985 274 155 0 274 85 55

55.3 30.9 47.0 52.7 46.0 58.1 ± 38.0 21.2 36.4

824 1620 671 466 148 65 29 170 67 35

36.5 9.7 18.6 50.2 47.3 56.9 ± 17.6 6.0 ÿ14.3

523 1463 546 232 78 28 64 140 63 40

limited by di€usion, mixing, bio®lm thickness and availability of substrate. It was suggested by Laursen et al. (1994) that the removal of nitrate in BAF takes place deeper into the bio®lm, as long as organic material is present. The depth to which oxygen can penetrate into the bio®lm is determined by the bulk liquid DO concentration, the di€usion rate and the zero-order intrinsic removal rate of oxygen. In the aerobic layer, nitrate does not take part in any reactions and di€uses through the bio®lm inactively. In the deeper layers, it is utilized by microorganisms for cell synthesis and growth. Laursen et al. (1994), Ryhiner et al. (1994) and van Benthum et al. (1998) have shown that BAF are capable of denitri®cation with oxygen present when treating municipal and industrial wastewater. According to Tijhuis et al. (1994), bio®lms develop when the hydraulic retention time is smaller than the reciprocal maximum growth rate of microorganism (HRT < 1=lmax ). If this criterion is met for nitri®ers as well as denitri®ers, both types of microorganisms will grow in the bio®lm (van Benthum et al., 1998). On the other hand, development of both nitri®ers and denitri®ers in the bio®lm will lead to overgrowth of the slowest growing organisms (autotrophic nitri®ers) by the fastest growing (heterotrophic denitri®ers), if both kinds of organisms are supplied with substrate, as demonstrated experimentally by Zhang et al. (1995) and van Benthum et al. (1997). In principle, growth of denitri®ers in suspension (at suciently long anoxic retention time) is more desirable than growth in bio®lms (van Benthum et al., 1998) because a heterotrophic layer growing over the nitri®ers in bio®lms increases the di€usion limitation. This decreases the DO concentration and, consequently, limits the nitri®ersÕ conversion of ammonium. Alkalinity is produced in denitri®cation reactions and the pH is generally elevated, instead of being depressed as in nitri®cation reactions (Randall et al., 1992). It was observed in the present study that there was an increase of pH in the ®rst bio®lter (from an average of 7.6 in the in¯uent to 8.3 exiting the bio®lter) and also a slight reduction in alkalinity (about 10%). Exiting the second bio®lter, the average pH was lower (average of 7.9) than

Overall bio®lter (% Red.) 71.6 37.6 56.9 76.4 71.5 81.9 ± 48.9 25.9 27.3

exiting the ®rst bio®lter and alkalinity was reduced by 42%. Although the lack of pH decrease in the ®rst bio®lter is not clear evidence of simultaneous nitri®cation/ denitri®cation in the bio®lters, it implies that the loss of nitrogen may be due to denitri®cation as well as ammonia volatilization. Total-P and OPO4 -P reductions were less than nitrogen and COD reductions. Overall reductions were 26% for Total-P and 27% for OPO4 -P (Table 2). The backwash removed a signi®cant portion of this phosphorus reduction as discussed in Section 3.6. 3.2. Nitrogen transformations and temperature e€ects The relative concentrations of NH3 -N and NO2 + NO3 -N in and out of each bio®lter during the year are shown in Figs. 2 and 3. The NH3 -N reduction in BIO1 was much less when temperature was low. Corresponding to this, BIO1 e‚uent had much lower NO2 ‡ NO3 -N when temperature was reduced. Although nitri®cation occurred in BIO2 during low temperature, the BIO2 e‚uent often had reduced NO2 ‡ NO3 -N during lower temperatures compared to higher temperatures (Fig. 3). Nitrite nitrogen was also measured and was sometimes more than 30 mg/l in

Fig. 2. Relative concentrations of NH3 -N and NO2 + NO3 -N in and out of bio®lter 1 (BIO1) (June 1997±May 1998).

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Fig. 3. Relative concentrations of NH3 -N and NO2 + NO3 -N in and out of bio®lter 2 (BIO2).

e‚uent from either bio®lter. This indicates that the Nitrobacter may be limited in some way resulting in incomplete conversion of nitrite to nitrate. The temperature e€ect on TKN reduction through the two bio®lters combined is shown on a monthly average basis in Fig. 4. The monthly average TKN reduction percentage varied from more than 80% at 25°C to less than 40% at 10°C (Fig. 5). Because the loading rate was not constant each month, the TKN reduction was also plotted against loading rate of TKN (not shown), but showed no signi®cant correlation. Thus, temperature was considered the main e€ect causing lower TKN reductions in November through March. Linear regression of TKN percentage reduction against temperature for all sampling days had an R2 of 0.67 (Fig. 4). 3.3. Phosphorus reductions The Total-P concentrations in and out of each bio®lter during the year are shown in Fig. 6. Most of the phosphorus reduction occurred in BIO1. The ``Total-P

Fig. 5. Temperature e€ect on mean monthly TKN reduction through the two bio®lters combined.

Fig. 6. Concentrations of Total-P in and out of the bio®lters.

in'' was reduced somewhat during November through March and may re¯ect less Total-P in liquid recycled from the lagoon for ¯ushing. However, the lagoon liquid was not sampled frequently enough to know whether this was the case. Overall, the bio®lters are expected to mainly reduce phosphorus by incorporating it in bio¯oc that is removed with backwash. The overall reduction in Total-P was 26% (Table 2). Increased phosphorus reductions could be obtained by chemical precipitation or possibly by having alternating periods of aeration and non-aeration to promote biological luxury uptake of phosphate as described by Sammut et al. (1994) and Goncalves et al. (1994). 3.4. COD reductions

Fig. 4. Linear regression of TKN percentage reduction against temperature.

The COD concentrations in and out of each bio®lter during the year are shown in Fig. 7. Most of the COD reduction occurred in BIO1. This was expected because heterotrophic bacteria should reduce most of the oxygen demand from carbonaceous material in BIO1. However, having a second bio®lter did result in further reduction

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187

by season was the lower reductions for most parameters in December 1997±March 1998, when lower temperature occurred. The December 1997±March 1998 percentage reductions for all parameters, with the exception of Total-P and OPO4 -P, are signi®cantly di€erent from all other periods at the 5% level. The April±May 1998 percentage reductions are highly variable (note large con®dence intervals for all parameters with the exception of NH3 -N) and are not signi®cantly di€erent from other periodÕs percentage reductions at the 5% level for COD, VS, SS and Total-N. It appears that there are no seasonal e€ects on percentage reductions of both TotalP and OPO4 -P. Fig. 7. Concentrations of COD in and out of the bio®lters.

of COD. The COD percentage reduction on a monthly basis was lower at lower temperatures but did not average below 50% for any month. The relationship between COD loading rate and COD removal is shown in Fig. 8. A linear regression of the data indicated that COD removal rate was linear with loading rate (slope of 0.84) and had an R2 of 0.92. 3.5. Reductions by season In order to further examine if reductions by the bio®lters varied much by season and with loading rate, data were averaged for ®ve periods (Table 3). The loading rate was increased in September by about 50% to test the response of the bio®lters. The percentage reductions in concentrations dropped somewhat in September compared to June±August 1997, but with a higher loading rate, the mass removal would be greater in September. The 95% con®dence intervals of the means are also shown in Table 3 to indicate the variation in percentage reduction for each time period. The most obvious e€ect

Fig. 8. Relationship between COD loading and removal (not including backwashes).

3.6. Mass balance Using ¯ow rates, concentrations, and volumes of backwash, a mass balance was calculated for COD, VS, SS, Total-N and Total-P (Table 4). On the average, nearly 20% of the in¯uent volume went to BIO1 backwash and approximately 8% went to BIO2 backwash. Nearly 30% of the COD, VS and Total-N were removed with BIO1 backwash, while about 10% of each were removed in BIO2 backwash. On average, about 15±40% of the various parameters remained in the e‚uent from BIO2. About 25±35% were lost (unaccounted for in the mass balance), except for Total-P, which showed more than 100% recovery, probably because of errors in sampling. Taking one grab sample twice a week or once every two weeks during some periods, may not be adequate frequency to re¯ect the true average condition. The calculation of mass balances indicates that more than one-third of the nitrogen and more than one-half of the phosphorus were removed in the backwash, and thus the management and utilization of the backwash is an important part of the waste management system. 3.7. Odor reductions Liquid samples were taken for evaluation by an odor panel on four di€erent dates: 4 August 1997, 21 October 1997, 9 February 1998 and 23 March 1998. Average results with signi®cant di€erences between samples are shown in Fig. 9. There were signi®cant reductions in odor intensity from about 5.6 to about 2 and odor irritation from about 4.7 to less than 2 in the bio®lter e‚uents, with most of the reduction taking place in BIO1. Thus, for odor control, the advantage of the second bio®lter is not obvious, but that depends on what criteria are acceptable. Also, if the e‚uent is stored for a period of time, there could be greater di€erences in odor from BIO1 e‚uent and BIO2 e‚uent. The odor qualities of the BIO1 and BIO2 e‚uents were between 4 and 5, where 4 is a neutral quality and numbers higher than 4 are unpleasant. Routing the e‚uent through a settling/

Mean and 95% con®dence interval of the mean.

19.2 24.2

5.2 6.6

a

5:2  0:4a

9:7  0:7 5:7  0:8 5:3  0:5 1:4  0:2 0:5  0:1 1:1  0:2

Volume (m3 /day) 2:7  0:6 1:9  0:6 2:6  0:5 0:4  0:1 0:2  0:1

Mass (kg/day)

Backwash from Bio®lter 1

Mass (kg/day)

In¯uent

34  8 31  8 54  14 27  6 48  14

Mass of in¯uent (%)

44  35 50  7

56  8 ± 42  18 33  7

VS

69  27 72  5

79  5 61  17 79  12 45  16

SS

0:4  0:1

Volume (m3 /day)

0:7  0:1 0:5  0:1 0:7  0:2 0:11  0:04 0:06  0:03

Mass (kg/day)

Backwash from Bio®lter 2

64  26 69  3

72  4 67  5 70  7 55  6

87  5a ± ± ± ± ±

COD

BOD

Percentage reduction by bio®lters

Volume (m3 /day)

Mean and 95% con®dence interval of the mean.

COD VS SS Total-N Total-P

Parameter

Table 4 Flow volume and mass balance for selected parameters

a

27.6 24 20.5 10

5.7 9.6 7.9 5.1

June±August 1997 September 1997 October±November 1997 December 1997±March 1998 April±May 1998 June 1997±May 1998

Average temperature (°C)

Average COD loading (kg/m3 /day)

Period

Table 3 Seasonal e€ects on reductions by the combined bio®lters

82 10  2 13  2 82 13  3

Mass of in¯uent (%)

81  14 70  7

82  9 76  8 67  20 46  12

TKN

3:7  0:3

Volume (m3 /day)

E‚uent

95  5 82  6

92  8 82  8 76  15 50  13

NH3 -N

2:2  0:2 1:2  0:3 1:0  0:2 0:5  0:1 0:25  0:04

Mass (kg/day)

50  32 46  7

58  10 55  8 48  19 25  8

Total-N

25  3 16  4 20  3 39  5 56  6

Mass of in¯uent (%)

14  29 24  7

27  12 12  22 35  6 25  7

Total-P

32  10 35  9 28  14 24  10 ÿ12  15

Unaccounted (%)

10  43 22  14

6  23 29  17 47  20 49  19

OPO4 -P

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189

Fig. 9. Odor perception results. Letters denote statistical signi®cance (P < 0:05) for odor intensity, odor irritation and odor quality.

polishing tank labeled DEN1 did not signi®cantly a€ect the odor. The backwashes from BIO1 and BIO2 were sampled for odor on the last three dates. BIO2 had lower odor ratings than BIO1. Both backwashes had higher odor intensity, irritation, and unpleasantness than the e‚uents. If the backwash material were stored for a period of time, the odor could increase further. The backwash also has the capacity to be further settled and supernate removed. Thus, depending on how the backwash is managed, further analysis of odor potential may be needed. 4. Conclusions Two up¯ow aerated bio®lters in series were able to signi®cantly reduce odor intensity and irritation as evaluated by an odor panel using liquid in¯uent and e‚uent samples. Backwash from the bio®lters had higher odor intensity and irritation than the e‚uent. Odor also needs to be evaluated for backwash and ef¯uent which have been stored for a period of time. With average loading over 12 months of 6.6 kg COD/m3 /day of media, reductions in concentrations from in¯uent to e‚uent were 72% for COD, 57% for VS, 76% for SS, 72% for TKN, 82% for NH3 -N, 49% for Total-N and 26% for Total-P. The reductions were a€ected by temperature, with higher reductions at higher temperatures. Most of the reductions in concentrations occurred in the ®rst bio®lter, but the second bio®lter still had signi®cant reductions as percentage of in¯uent concentrations to the second bio®lter. During low temperature, the ®rst bio®lter had very little nitri®cation, demonstrating some advantage for having the two bio®lters in series for nitri®cation at low temperature. The COD mass removal rate was linear with loading rate over the range of about 2±12 kg COD/m3 / day of ®lter. Calculating a mass balance over the period of monitoring, approximately 30% of the in¯uent volume, 35% of the Total-N and 60% of the Total-P were removed

with the backwash from the bio®lters. The backwash has potential to yield further settling and concentration of the biosolids. Management and utilization of the backwash is an important consideration for implementing this system on farms. Another potential result of having an aerobic treatment system that converts organic and ammonia nitrogen to nitrate is that denitri®cation can occur when e‚uent is recycled to the swine buildings for pit recharge (``pull-plug system''). In this system, 30±45 cm of liquid depth is added to the pit below the slotted ¯oor and collects manure for usually one to two weeks before emptying. Denitri®cation in the pit could be an advantage if the farm needs to reduce nitrogen.

Acknowledgements The funds for evaluation were provided by the North Carolina GovernorÕs Oce through the Department of Environment and Natural Resources and the North Carolina Agricultural Research Service (Dr. Johnny Wynne, Director) and the North Carolina State University Animal and Poultry Waste Management Center (Dr. Mike Williams, Director). Funding of project, equipment and technical support were also furnished by Ekokan (Ms. Alexandra Kantardjie€, President). Thanks go to others who helped with the evaluation: Yvon Grenier of Ekokan, Doug Williams (NCSU Engineering Research Technician), the NCSU Environmental Analysis Lab. (Rachel Huie, Chris Hayes, and Indira Thillai), Dr. Kelly Zering (Agricultural Resources Economics Department), Dr. John Classen (Biological and Agricultural Engineering Department), Dr. Mike Williams who coordinated the odor evaluation with Dr. Susan Schi€man (Duke University). Installation and site support were also provided by the North Carolina Agricultural Research Service Lake Wheeler Road Field Laboratory sta€ (Ken Snyder, Superintendent and Tom Ste€el, Swine Education Unit Research Manager and their sta€).

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