What are process controls for treating wastewater in the dairy industry?

Aug. 5, 2014

Dairy wastewater has a high organic load, requiring biological treatment for permit-compliant discharge. Successful, consistent biological treatment in turn requires stringent controls on primary treated …

Dairy wastewater has a high organic load, requiring biological treatment for permit-compliant discharge. Successful, consistent biological treatment in turn requires stringent controls on primary treated wastewater, preceding dairy-waste biological treatment. Together they are typically adequate for compliant discharge into sewer authority collection systems. What treatment is used depends on the waste-load volume and concentration.

Raw milk is processed at dairies, either for immediate consumption or converted into products such as whey (protein powder), cheese, butter, dried milk powder and ice cream. Dairies handling milk are classified as receiving, bottling, condensing and manufacturing dry milk powder & ice cream and making butter & cheese.

Milk product values for biochemical oxygen demand can range as follows:

Whole milk: 85,000-110,000 Mg/L BOD

Skim milk: 60,000-80,000 Mg/L BOD

Buttermilk: 50,000-70,000 Mg/L BOD

Whey: 20,000-40,000 Mg/L BOD

Production in a dairy is batch type, i.e., one batch of milk is processed, the equipment is drained, thoroughly cleaned and the next batch is taken for processing. Because the flow of waste water from a dairy comes in slugs, a hydraulically loading wastewater treatment is a distinct possibility.

The amount of water used in a dairy, which can generate average wastewater ranges between one and six gallons for every one gallon of milk processed, is the source of the hydraulic load. Most of the load is related to required sanitation, including equipment cleaning and washing floors.

The characteristics of dairy waste can range as indicated below:

BOD 500 – 3,000 Mg/L

COD 750 – 7,500 Mg/L, i.e., chemical oxygen demand

TSS 500 – 1,500 Mg/L, i.e., total suspended solids

O&G 200 – 1,000 Mg/L, i.e., oil & grease

Other dairy-waste characteristics include lactose, protein, fat, oil and grease presence; hydraulic overloads due to water dumps or wash water; variable pH levels as sanitizing chemicals affect wastewater treatment; wastewaters may be generated at receiving stations, pasteurization and   homogenization; washing and sanitizing are important and wastewater sources include spills and spoiled product.

The COD and BOD relationship is favorable for applying biological treatment processes following certain stringent preliminary and primary treatment processes.

Preliminary and primary

Equalization reduces hydraulic overloads and the possibility of flow-variation shock loads. Equalization ensures a reasonably uniform volume and composition flow, available for discharge to each subsequent wastewater treatment.

Equalization process control may be limited to available volume capacity. Equalization capacity requirements change as incoming wastewater characteristics change. Mixing increases the effectiveness of equalization.

The pH, as an expression of base or acidic intensity of wastewater is mathematically expressed as the logarithm (base 10) of the reciprocal of hydrogen ion activity. Changing the ion activity of wastewater causes certain compounds in wastewater to either be in solution as soluble or convert to a physical form.

Adjusting pH, known as neutralization, is adding an acid or alkali (base) to wastewater, causing the pH of to go up or down to accommodate a treatment process. Process controls establish an optimum pH to perform a treatment process, for chemicals use and for equipment and instrumentation to consistently control pH.

Flow is the movement or velocity of wastewater, measured typically in gallons per minute. Flow control establishes and maintains a desired flow rate for pumping wastewater, chemicals or sludge. It starts with determining the applications’ desired flow rate and having pumps and instrumentation to consistently maintain those rates.

Coagulation and flocculation

Coagulation is the clumping together of very fine particles into larger particles (floc) caused by the use of chemicals (coagulants).  Chemicals neutralize the electrical charges (ions) of the fine particles, allowing them to come closer and form larger clumps.  This clumping together makes it easier to separate solids from the wastewater by settling, skimming, draining or filtering.

Flocculation is the gathering together of fine particles after coagulation to form larger particles by a process of gentle mixing.

Controlling coagulation and flocculation begins by selecting chemicals appropriate for causing floc to separate from wastewater. Controlling coagulation and flocculation includes injecting appropriate chemicals in a flow stream at the optimum flow rate with specified mixing velocity and adequate holding time for floc separation.

Clarification reduces color and suspended-matter concentration in wastewater. Clarification allows consistent separation of floc from clarified wastewater.

Flotation removes oils and greases, separates physical solids and decreases BOD associated with physical solids, using a dissolved air flotation (DAF) clarification process unit. Doing so ensures consistent floating of floc from clarified wastewater.

Sedimentation is the settling of physical solids to the bottom of a solids separation or clarification process unit. It allows the consistent settling of floc from clarified wastewater.

Secondary or biological treatment

Activated Sludgeviaextended aeration uses an aeration basin, secondary clarifier and sludge-holding or digestion-process units. Controlling activated sludge includes maintaining acceptable continuous primary treated wastewater entering the activated sludge process and maintaining activated sludge processes to discharge compliant secondary or final clarifier effluent.

Treating wastewater in the dairy industry has been described above, however if you have specific issues treating dairy waste or other wastewater queries, please submit a question.

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