An In-House Dairy Wastewater Treatment Plant
The dairy industry uses massive amounts of water to processes raw milk for dairy produce and generates roughly 3 L of wastewater per 1 L of processed milk. It is one of the most polluting of industries, not just in terms of the volume of effluent but also in terms of the composition. Commercial dairies have two options – to pretreat and pay to discharge to municipal sewerage or to operate an in house treatment plant. However, there is no one size fits all solution. There are multiple possible configurations for a dairy wastewater treatment plant, selecting the correct one can be a challenge – but why?
In terms of biodegradability, dairy process wastewater is complex – it contains a combination of easily degradable carbohydrates and not so easily degradable proteins and fats. It is variable in pH also, rapidly changing from alkaline to acidic when lactose ferments to lactic acid. Dairies are multi product factories and the contents of pollutants in the wastewater will change with the start of each new cycle in the production process. This lack of consistency needs to be addressed by the treatment system.
Technologies Used in Dairy Wastewater Treatment
Dairy wastewater needs complex treatment prior to discharge in order to prevent environmental damage. This is due to the high concentration of organic materials including protein, carbohydrates, fats, grease and minerals that elevate BOD. Moreover, dairy factory cleaning processes generate wastewater containing detergents and cleaning agents that increase COD. Although there are many ways to reduce BOD and COD, biological treatment is the main method using aerobic, anaerobic or a combination of both technologies within a dairy wastewater treatment plant.
Steps in the Treatment of Dairy Wastewater
Mechanical treatment of dairy wastewater involves filtering out suspended solids with a mechanical screen. This reduces the organic load and protects the subsequent treatment equipment from blockages. An equalization/buffer tank will hold 6 – 12 hours of influent in order to smooth fluctuations in the flow. Supplying air at this stage helps to mix the wastewater and regulates the consistency for the next treatment stage. FOG removal usually follows flow balancing in a conventional dairy wastewater treatment plant.
Chemical treatment, also known as precipitation, removes colloids and soluble contaminants of dairy wastewater. This stage includes reagent oxidation or pH correction. Dissolved air flotation reduces organic loading with coagulants (Al2(SO4)3, FeSO4 and FeCl3) and flocculants. Here, controlling the pH is necessary to achieve the best conditions for coagulation (an acidic environment). However, the pH must be adjusted back to neutral levels before the next stage of treatment or can upset activity of microorganisms.
This completes the pretreatment of dairy effluent. It may be possible for a dairy to discharge suitably pretreated wastewater to a municipal treatment plant, with the approval of the relevant authority.
Biological Treatment Systems for Dairy Effluents
Biological treatment removes remaining impurities with the help of microorganisms. Aerobic systems use oxygen to breakdown the organic matter. Within the treatment system, air blowers supply the wastewater with oxygen, allowing the bacteria to multiply continuously. This process results in the formation of an activated sludge or biomass and the separation of clarified effluent. Most of the biomass is recirculated to maintain the biological process.
Dairy wastewater treatment plants often use an SBR system for aerobic treatment, due to its effluent flexibility and loading capability. The resultant effluent is ready for reuse or discharge, the excess sludge requires further treatment. Other aerobic systems include, fixed bed reactors, rotating biological contactors, trickling filters and moving bed biological reactors.
Anaerobic systems, on the other hand, do not require oxygen in order to break down organic matter and as such do not have the high energy requirements associated with aeration in aerobic systems. Other advantages of anaerobic digestion include the production of biogas (an energy source) and less sludge production. An example of a suitable anaerobic system for a dairy wastewater treatment plant is an anaerobic fixed bed reactor (AFBR) due to its capacity for microorganism retention and ability to cope with influent variations/shock loads.
However, anaerobic treatment is only suitable as a preliminary step in the biological treatment of dairy wastewater due to its weak effect on nutrient removal. It must be combined with a localized polishing step. Once aerobic/anaerobic biological treatment is complete, the residual sludge is sent for sludge treatment and disposal.
Cost Savings Associated with In-House Dairy Wastewater Treatment Plant
A study conducted by the Association of German Dairying in 2010 revealed that the costs associated with operating an in-house wastewater treatment plant were up to to 2/3 less than those of direct dischargers i.e. users municipal treatment plants. Ultimately, the most suitable treatment system depends entirely on the process and produce of the individual dairy. The costs associated with anaerobic systems are generally less but such systems are not suitable for all types of dairy effluent. The dairy processing handbook advises to contact local authorities at an early stage when planning a new plant to discuss discharge consents.
A reputable wastewater treatment company will be able to conduct the necessary tests in order to provide the most suitable and cost effective treatment system for a commercial dairy. A pilot scale system can be tweaked and tailored to suit the treatment requirements of a particular dairy before a full scale treatment system is put into place. Remember – every dairy is different!