Biological Treatment of Dairy Manure Using
Sequencing Batch Reactors: Improving
Profitability Through Innovative Design

Paul C. Reeves, Kerstin P. Johnson, and Carlo D. Montemagno,
Department of Agricultural and Biological Engineering
Cornell University

Introduction

The Sequencing Batch Reactor (SBR) displays great potential for increasing the profitability of the dairy industry by eliminating environmental problems associated with manure disposal. Recent trends in dairy farming have demonstrated how higher profitability can be achieved through greater herd sizes. This is facilitated by new levels of specialization with feed inputs supplied by outside farms and industries. This allows dairy farms to increase their herd sizes without acquiring new land as they focus solely on milk production. However, as herd sizes increase relative to farm acreage, land application of dairy manure may no longer be a feasible disposal strategy due to the potential for adverse environmental impacts. Excessive land application of manure can cause nutrients such as nitrogen and phosphorous to leach through the soil and contaminate ground water supplies. Surface runoff can also dump excessive amounts of nutrients into streams and lakes causing problems with municipal drinking water supplies and resulting in heightened fish mortalities due to eutrophication. Recent outbreaks of water-borne pathogens such as Cryptosporidium, Giardia and Pfiesteria have also been linked to runoff associated with agricultural waste. Strong odors, especially during the spring thaw, also pose significant local problems with zoning, local permitting, and community relations. In light of the economic forces promoting larger her sizes, combined with the potential for environmental problems, innovated and cost-effective strategies are needed to handle dairy waste.

Biological treatment of organic waste using activated sludge is a proven technology used in municipal sewage treatment facilities. Conventional aerobic treatment processes have been used to reduce the potential pollution impact of liquid manure, but these processes have not been successful in reducing both carbon and nitrogen compounds to satisfactory levels at a reasonable cost. However, an innovative design known as the Sequencing Batch Reactor minimizes capital costs by incorporating both aerobic and anaerobic processes in a single reactor [See Irvine and Ketchum, 1989, for a review of the SBR method]. This allows for diverse and flexible operating conditions suitable for the stabilization of carbon, nitrogen, and phosphorous rich compounds. Studies have already been conducted using SBR technology to treat municipal waste in the U.S [e.g., Irvine and Ketchum, 1982], as well as high strength agricultural wastes such as liquid swine manure in Canada [e.g., Fernandez, 1994; Fernandez et al., 1991; Fernandez and McKyes, 1991; Lo et al., 1991], and veal waste in Europe [e.g., Willers et al., 1993], several of which demonstrated high removal rates of COD, Nitrogen, and Phosphorous. The application of this efficient technology the biological treatment of dairy manure is an attractive alternative to land application of raw manure or storage in waste lagoons. By incorporating SBR technology into the dairy farm waste management scheme, farms can continue to expand to improve the profitability while also reducing the environmental and health risks associated with agricultural wastes.

Two bench-scale reactors are currently being operated in laboratory facilities at Cornell University to measure the nutrient neutralizing capacity of the SBR method when applied to the liquid fraction of dairy manure. At present, both reactors are being operated under the same parameters in order to ensure reproducibility of results. The reactors have a working volume of 3.0 liters and are being operated at a hydraulic retention time of 10 days under ambient laboratory temperatures (~ 21șC). The cycle time of the reactors is 24 hours and consists of five distinct operational modes as outlined in Table 1. Reductions in COD are achieved under aerobic conditions. In addition, aeration promotes the conversion of ammonia to nitrate (nitrification). Nitrate is further converted to nitrite and eventually nitrogen gas under anoxic conditions (denitrification). The sequencing and duration of these periods are of critical importance for efficient nutrient removal.

The reactors are fed 100 ml liquid manure diluted in 200 ml of distilled water daily using peristaltic pumps. The feed rate is 3.8 ml/min giving a total feed time of 80 minutes. The aerated portion of the cycle time is currently being operated at 3.0 liters/min of compressed air giving a dissolved oxygen concentration of approximately 5.0 mg/liter. Mixing is accomplished using mechanical mixers. Measurements of pH have consistently ranged between 8.8 and 9.0.

Effluent supernatant and mixed liquor are wasted by hand using pipettes. The effluent is then centrifuged and filtered through a Whatman Type A/E glass fiber filter prior to performing chemical analyses. COD and Nitrate concentration measurements are performed using closed reflux digestions while Ammonia/Ammonium concentrations are measured using ion-specific electrodes. All chemical analyses are performed using methods outlined in Eaton et al. [1995].

The reactors are currently in a start-up mode to allow the biological populations to adapt to the dairy manure substrate, so the results presented in this section are preliminary. An adapted population will be indicated when effluent concentrations are demonstrated to be stable and when the concentration of biological solids can be predicted. This will allow a solids wasting strategy to be determined so that a constant suspended solids concentration can be maintained. At present the concentration of suspended solids is slowly declining by approximately 2% daily.

Preliminary results from the bench-scale reactors indicate that the liquid fraction of dairy waste can be significantly degraded using SBR systems. Figure 1 shows a time series of COD concentrations and removal rates in the treated effluent for days 35-46 of the start up period. The total solids concentration on day 43 (December 19) was 17,388 mg/liter ±100 mg/liter. Given that the influent COD concentration was approximately 75,000 mg/liter, these results indicate a reduction of over 97%. The improvement of removal rate with time demonstrates that the population is evolving to be more efficient in utilizing COD. Given that the total solids concentration declines over the same time period, even higher removal rates can be expected in the future.

Preliminary results indicate that effluent ammonia concentrations are declining to approximately 2.5 mg/liter, compared to an influent concentration of approximately 1,700 mg/liter. These high removal rates (>99%) may be indicative of ammonia stripping since we have not yet demonstrated that biological nitrification/denitrification is responsible for these results. However, we have not detected any ammonia smell coming from the reactors and other researchers demonstrated that air stripping of ammonia was negligible at the pH values similar to those observed in our reactors [Fernandez, 1994].

Our results demonstrate that the SBR system is effective in significantly reducing the primary nutrients of dairy manure. As well the system also serves to minimize odor problems. We are currently working on measuring the significant biological and chemical reaction rates required for modeling the nutrients in the system. This will allow us to modify the sequencing strategy with predictable results so as to optimize the performance of the reactors.

Another current research focus is to determine accurate biological yield coefficients so that an effective solid wasting strategy can be developed. Optimal conditions require that the sludge grown during a 24-hour cycle be extracted from the reactor during the waste period so as to ensure quasi-steady state solids concentrations. Since all biological reaction rates are dependent on the biomass concentration, this is a priority concern.

The ultimate goal of our project is to design a field scale SBR capable of handling the waste of 500 dairy cows. We aspire to develop a robust system capable of reducing COD, Ammonia, and Phosphorus levels by 98% or better. These goals require the development of an optimal set of operating conditions, which will be determined by the laboratory measurements. A complete economic analysis of the method including capital and operating costs will also be developed in order to analyze the economic potential of the method.

Table 1: Operational Sequencing Parameters of the Sequencing Batch Reactors

 

Operation	On	Off
Mixing	10:00 a.m.	8:00 a.m.
Idle	8:00 a.m.	10:00 a.m.
Wasting	9:45 a.m.	10:00 a.m.
Fill	10:00 a.m.	11:20 a.m.
Aeration	12:00 p.m.	6:00 a.m.

 
Figure 1: COD Concentrations and Removal Rates for Days 35-46 of the Start-Up Period. Reactor A.

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