Milk house only characteristics
The second critical input into the system design is the wastewater characterization. Depending on the specific system, some of the wastewater constituents are more important than others. For instance, Biochemical Oxygen Demand (BOD5) concentrations are critical for sizing an aerobic treatment system and soil infiltration area but are not critical in a surface irrigation system. Rather, the sizing of the irrigation application area is based on the amount of nitrogen and/or phosphorus in the wastewater.
Milk house wastewater strength (concentration of organic material, solids, nutrients, and fats) is quite variable as it leaves the milk house and is a function of the time of day and wash cycle. As such, treatment system designs are based on the wastewater concentration as it leaves the septic tanks. In all treatment system designs there are one or two septic tanks installed prior to the final treatment. The waste strength leaving the septic tanks is a function of concentrations in the raw wastewater from the milk house and the Hydraulic Retention Time HRT in the septic tanks. Table 1 shows the typical concentrations of milk house wastewater leaving the first and second septic tanks.
The BOD5 concentration is the primary waste parameter used in the design of Bark Beds, Aerobic Treatment Units (ATUs), and Recirculating Media Filtration Systems (RMFs). Nitrogen and phosphorus loading (lbs produced per year) are the main parameters used for the design of irrigation systems. Total Suspended Solids (TSS) and Fats Oils and Grease (FOG) are some other general measures that quantify the strength of the waste and are used with other wastewater treatment system designs.
Effluent and flow sampling from two parlor systems was collected monthly over a two year period. One of the farms was averaging about 11 gallons per cow per day while the other was averaging about 7 gallons per cow per day. Concentration and mass loading data suggest this difference is primarily dilution water (additional water used in washing.) Average BOD5 concentration for the two sites was 955 and 1600 mg/L, P concentrations were 22 and 36 mg/L and total nitrogen concentrations were 125 and 190 mg/L. Mass loading on a per cow per year basis was similar between sites with values of 31 lbs/c/d BOD5, 0.7 lbs/c/d P, and 4 lbs/c/d TKN. This sample data was from samples coming out of the second septic tank.
CAUTION: On a dairy farm there are situations when there is waste milk that cannot be added to the bulk tank (milk from fresh and treated cows) or there is a bulk tank failure or spill. Since milk has a BOD5 concentration of 100,000 mg/L (Wright et al., 1998), the addition of waste milk to the wastewater treatment system will significantly overload the aerobic treatment systems and bark beds. Typical milk house waste contains effluent approximately 2 to 3 gallons of residue milk per day generated during the washing of the equipment, pipeline, and bulk tanks. Depending on the situation, waste milk from fresh or treated cows would add two to five times that amount of milk per day to the wastewater treatment system, significantly impacting the organic loading and system performance. As such, all waste milk should be kept out of the milk house wastewater treatment system and disposed of with the manure.
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