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July 2008

Aerobic treatment units at MSU dairy farm

Potential of Two Aerobic Units to Treat Milking Facility Wash Water

Milking facilities generate 3.5 to 11 gallons of wash water per cow daily. This large volume of wash water requires proper treatment and disposal practices to prevent potential negative environmental impacts. Current practices including manure storage lagoons, land application, and use in alternative farming facilities are effective but not perfect. This article describes the use of aerobic treatment units as an alternative.

Becky Larson
Steven Safferman
Dept. of Biosystems and Agricultural Engineering

Dairy milking facilities produce wash water, a high strength waste, as a byproduct of cleaning the milking facility after each milking event to maintain sanitary operations. Wash water composition includes high concentrations of cleaning products, fresh water, milk waste and animal waste. Wright and Graves report a wash water production range of 3.5 to 11 gallons/cow per day (1). Variation in production rates is largely due to management practices, number of milking events per day and herd size. The National Agricultural Statistics Service reports 344,000 head of cattle in Michigan alone, and over 9 million cattle in the United States (2). Consequently, the volume of milking facility wash water can be calculated at 400 million to 1.3 billion gallons annually in Michigan with a range of 11 billion to over 36 billion gallons annually in the US. More details concerning the characteristics of this wash water can be found in a prior MDR article by Safferman (3). This large volume of wash water requires proper treatment and disposal practices to prevent potential negative environmental impacts.

Current practices for management of the milking parlor wash water include storing it in manure storage lagoons, land application and use in alternative farming facilities, such as composting. Manure storage lagoons provide little to no treatment. Land application at acceptable nutrient agronomic rates is the major disposal option but requires extensive land management planning and is restricted by land availability and climate. The use of wash water in alternate farming facilities, such as composting, rarely requires the volume of water produced by the milking facility. Further, many farms do not operate these alternative facilities. Wash water can account for 20-50% of manure pit storage volume (4). This increase in liquid content results in larger volumes of waste, increasing the risk of leaks, overflows, runoff and migration of undesirable solids and nutrients into groundwater. Further, this wash water has minimum nutrient content, yet there is a high cost for moving this excess water to the cropland.


This research examined an alternative treatment and disposal technique for dairy wash water. Specifically, two aerobic treatment units with different solid/liquid separation techniques were evaluated for their capability of treating high strength wastes to a suitable effluent quality for reuse as non-contact first flush water. Reuse quality for a first flush scenario is ultimately determined by the farmer as there are no regulatory standards which define this value. Table 1 provides some recommended standards.

Design/Method and Materials

Experimentation was conducted at the Michigan State University Dairy Teaching and Research Farm. This facility actively milks between 140 and 160 dairy cows twice a day. Two treatment systems modeled after those used for onsite wastewater were tested. Each included shared primary settling tanks, individual dosing tanks to control flow, the aerobic treatment unit (ATU), a recirculation system for dilution of the ATU influent, and a ultra violet (UV) disinfection unit. Wash water was pumped from the milking facility collection pits beneath the milking facility to the treatment system settling tanks. After the settling tanks, separation of flow was achieved using a distribution box to maintain two treatment lines, one for each of the aerobic treatment solid/liquid separation designs. The ATUs were the NayadicTM and the Multi-FloTM. Both treatment units (explained in more detail below) are off-the-shelf designs for onsite generated domestic household waste and are manufactured and provided by Consolidated Treatment Systems Inc. Following the ATUs, recirculation tanks diverted a portion of treated wash water back to the dosing tanks to dilute the primary effluent to reduce the ATU influent organic concentration. Specifically, for every part discharged from the system, 3 parts (by volume) were recirculated.

The ATUs were designed for a household flow of 750 gallons per day. An equivalent hydraulic loading for the milking facility was calculated as 68 gallons per day due to higher Biochemical Oxygen Demand (BOD) concentrations within the wash water as compared to household effluent. However, households typically produce slug loadings from the nature of domestic use (e.g. flushing a toilet) and do not typically recirculate as does the treatment system designed for the milking facility. For convenience, testing and data acquisition initiated with a dairy wash water flow of 50 gallons per day with the intent of treating higher flows, if warranted. Treatment units were initially filled with wash water and run until failure. Approximately three weeks before testing was discontinued, a second aerator was installed in the Multi-FloTM to increase the Dissolved Oxygen (DO) because of the low measurements indicating the system was oxygen limiting. The increased aeration resulted in a greater DO value which allowed for an increase in flow to 100 gallons per day. A variety of inline filters were tested for a short period in an attempt to reduce solids, but proved to be ineffective due to clogging.


Over the 6-month operational period, water quality was determined on a regular basis by measuring several parameters, those reported are in Table 2. Observations, trends, average values and reduction percentages were evaluated. A summary is provided in Table 2 for average values of the baseline influent (untreated wash water), NayadicTM effluent and Multi-FloTM effluent with the corresponding confidence intervals.

Baseline BOD and Chemical Oxygen Demand (COD) values were much greater than reported by literature. A tremendous reduction of the effluent BOD was realized, especially in the Multi-FloTM system. Chemical Oxygen Demand was substantially higher than BOD, indicating a significant amount of non-biodegradable carbon. Non-biodegradable carbon is a result of organic material which is not easily aerobically biodegradable, which would explain the higher COD values. The source of these materials is most likely cellulose materials such as fiber. Variation in the baseline values for BOD and COD were most likely due to the inconsistent daily volumes of milk waste added to the underground tanks. Manure was also a source of varying COD and BOD concentrations due to the random cleaning of an adjacent floor which was also connected to the underground tanks.

Levels of total and suspended solids were also tremendously high and variable. Only the Multi-FloTM was able to reduce the suspended solids substantially, however, a low level could not be sustained for long time periods. These results indicated a physical solid/liquid separation as provided by the socks in the Multi-FloTM was required for adequate treatment. The gravity solid/liquid separation mechanism in the NayadicTM was ineffective. The very high dissolved solids level undoubtedly contributed to the extreme baseline COD. Not only were high solids levels a problem in reaching treatment goals, they also caused major operational problems within all treatment segments.

Nitrogen (N) was monitored by measuring Total Kjeldahl Nitrogen (TKN), nitrate and ammonia. In this case, TKN was a measure of the organically bound nitrogen only, however it is typically known to include Ammonia. TKN (organically bound N only) represented about one-third of the total N in the wash water, while ammonia was the main remaining source. The effluent TKN was low, as it was most likely converted to ammonia by ammonification. Ammonification is dependent upon oxidizing conditions which were more typical in the Multi-FloTM. Effluent ammonia concentrations were also low, especially in the Multi-FloTM, most likely resulting from denitrification. Nitrate was not present in significant concentrations in any of the influent or effluent samples. This was not surprising as the high organic carbon levels and low Oxidation Reduction Potential (ORP) results in an environment conducive for denitrification of nitrate to N gas. Alkalinity and pH results substantiated these findings.

Phosphorus levels were unexpectedly high, most likely due to the large volumes of manure and possibly cleaning products that was mixed with the wash water. Any phosphorus removal can only be attributed to the particulate fraction being removed with solids.

Overall, as determined by DO and ORP levels in the aeration tanks, the system was typically oxygen limiting. However, a substantial improvement in water quality was realized throughout the testing. Treatment to a water quality level adequate for reuse could be realized in the Multi–FloTM system. However, these levels could not be maintained for greater than a month. Further, the flow rate of 50 gallons per day was substantially less than that produced at the dairy farm. The MSU Dairy produces an average of 1,800 gallons per day, or approximately 12 gallons per cow per day, above the reported values (1).


  • The following conclusions were reached from the testing research.
  • Aerobic treatment units proved able to treat high strength dairy waste wash water.
  • The Multi-FloTM consistently out-performed the NayadicTM for all water quality parameters.
  • The Multi-FloTM reached effluent water quality standards for reuse at a flow rate of 50 gallons per day for a first flush cleaning of the dairy milking facility for 1 month.
  • System maintenance was determined vital for proper treatment performance.
  • Treatment is reliant on the characteristics of the wash water produced by the farm. Large amounts of solids were determined to be the main detriment to system operation and treatment performance.
  • The volume of wash water treated, 50 gallons per day, does not meet production of the wash water requiring disposal. Feasibility of the treatment units will depend on improved efficiency.

The primary limitation of the system was the high solids levels in the influent wash water. A reduction in solids would lead to multiple positive improvements.

The potential to treat high strength wash water was shown. However, the use of an ATU-based system is still not proven because the high level of solids in the wash water at the MSU Dairy prevented sustainable operation. However, the results show that a similarly sized Multi-FloTM system may have potential for a smaller farm which practices solids reduction methods. Wash water characteristics closer to reported values and an increase to two aerators within the treatment unit would give the Multi-FloTM more potential as a viable treatment option.

The costs for these treatment units is reasonable to be considered for wash water treatment specifically at a smaller dairy. Because these units were not suitable for the testing location further tests aimed at answering questions concerning specific treatment volumes and their associated costs will be emphasized to determine practical implementation.


1. Wright, E. P. and R.R. Graves. Guideline for Milking Center Wastewater. NRAES-115 (DPC-15), Cornell University, Ithaca, NY, 1998.
2. USDA. National Agricultural Statistics Service. 2006. retrieved June 24 2008 from <http://www.nass.usda.gov/Data_and_Statistics/Quick_Stats/>.
3. Safferman, S.I. Milking Facility Wash Water: Facts and Figures. Michigan Dairy Review. 13(1): 1-2. January 2008.
4. Livestock Wastes Subcommittee, Midwest Plan Service, Livestock Waste Facilities Handbook, MWPS – 18, Second Edition, Midwest Plan Service, Ames, Iowa, 1985.
5. Dong, L., P. Y. Yang, P. S. Leung and C. N. Lee. 2003. Evaluation of Potential Dairy Farm Wastewater Treatment and Reuse Systems in the Tropics. Ninth International Animal, Agricultural and Food Processing Wastes Proceedings of the 12-15 October 2003 Symposium, 242-249.
6. USEPA Guidelines for Water Reuse. September 29, 2004. Technical paper no. EPA/625/R-04/108
7. Sarkar, B., P. P. Chakrabarti, A. Vijaykumar, and V. Kale. 2006. Wastewater Treatment in Dairy Industries – Possibility of Reuse. Desalination, 195, 141-152.





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