Got Milk? Got Wastewater Too! Treatment of Dairy Processing Wastewater
A major dairy processing operation owned and operated by the Michigan Milk Producers Association (MMPA) located in central lower Michigan was required to design and construct a process wastewater treatment plant within a 14 month period. The system is sized to treat up to 0.5 MGD of dairy processing wastewater at a biochemical oxygen demand (BOD) concentration up to 1,846 mg/L. Effluent from the treatment process is discharged to a relatively small receiving stream. The State of Michigan included very restrictive discharge criteria in the NPDES permit issued to MMPA for the new treatment process including a seasonal phosphorus limitation of 0.1 mg/L. Historical wastewater characteristics along with milk processing data from the plant were used to establish the treatment process basis of design. The treatment process was required to accommodate variable loadings due to seasonal and market fluctuations in the amount of milk received at the plant throughout the year. The treatment system incorporates preliminary equalization, activated sludge treatment using sequencing batch reactors (SBR), and secondary equalization prior to tertiary filtration. Selection, sizing, and procurement of the major treatment process equipment, prior to completion of the final design, was utilized to meet the aggressive schedule. This paper discusses the history of treatment alternatives used at the site, challenges involved in sizing a treatment system to accommodate a highly variable waste while meeting stringent discharge requirements, completing design and construction within an aggressive timeline, regulatory agency negotiations, and treatment system performance since start-up in early 2002.
MMPA is a cooperative of approximately 1,900 dairy farmers who produce about 60% of the milk in Michigan. MMPA’s Ovid plant has been in operation since 1957 and is located in the Village of Ovid, in rural central Michigan northeast of Lansing. The facility is the largest balancing plant in the state, with the capacity to process up to 3 million pounds per day of milk into commercial dairy products such as non-fat milk powder, bulk liquid dairy blends, and bulk
Wastewater treatment for the plant was accomplished using a spray irrigation system for land application of untreated wastewater since 1959. The wastewater was applied year-round to agricultural fields near the plant using a series of solid set sprinklers. The wastewater received treatment through site soils and seasonal vegetation prior to discharging to underlying groundwater and adjacent streams. In 1997, the Michigan Department of Environmental Quality (MDEQ) notified MMPA the current land application site was no longer a viable alternative for the treatment and discharge of the wastewater due to concern of maintaining surface water quality. The state was concerned that continued application would lead to degradation of stream quality due to nutrient enrichment, especially phosphorus. MMPA was required to investigate the extent of impact to the underlying groundwater and adjacent surface water, as well as develop a long-term solution for the treatment and discharge of the wastewater generated at the plant.
In 1998, MMPA secured a lease agreement to apply process wastewater to agricultural land located approximately three miles from the plant. The MDEQ granted a permit for year-round land application and groundwater discharge of untreated process wastewater. MMPA undertook the design and construction of a land application system to irrigate the process wastewater yearround. Construction of the system began in June 1999 and the system was operational in April 2000. This system consisted of eight center pivot irrigation machines applying wastewater to 132 acres. Concerns regarding odors and freezing during winter operations required MMPA to provide significant system modifications. To avoid the issues related to freezing during winter operations, it was decided to install a treatment system and pursue a surface water discharge permit under the National Pollutant Discharge Elimination System (NPDES).
A permit application was filed with the MDEQ, at which time it was determined the target receiving stream, the Maple River, was on the nonattainment list of water bodies for the State of Michigan. The cause for this listing was nutrient enrichment due to elevated background phosphorus levels in the receiving stream. The MDEQ initially set a very restrictive phosphorus limitation (0.1 mg/L) for year-round discharge to the river, but would consider a seasonal discharge to the river during the non-growing season at a maximum phosphorus concentration of 1 mg/L. The Maple River has a relatively low flow during the summer season in the area of the discharge. The Maple River flows used to develop effluent limitations in the permit were the estimated 95% exceedances flow of 6.1 cubic feet per second (cfs), a harmonic mean flow of 16 cfs, and a 90-day, 10-year low flow of 7.5 cfs.
To avoid the need to remove phosphorus to the proposed summer season level of, 0.1 mg/L, MMPA choose to pursue a blended permit. Treated effluent was targeted for crop irrigation from May through October and discharge to the Maple River for the balance of the year. Treatment system effluent would be directed to the river from November 1 through either April 1 or May 1 (depending on field conditions) and to the recently constructed spray irrigation fields during the remainder of the year. The combination of complying with both surface water and groundwater discharge limitations required a treatment system capable of removing the majority of incoming biological and chemical oxygen demand, conversion and removal of influent nitrogen to maintain a total inorganic nitrogen (nitrate + nitrite + ammonia nitrogen) concentration of less than 5 mg/L, and total phosphorus to less than 1 mg/L.