Dairy wastewater treatment plant
Wastewater treatment for dairy industry that uses milk as a raw material represents a problem that has no easy and immediate solutions. In almost all cases it requires a targeted and specific treatment if wastewater is not diluted with large volumes of domestic sewage.
The wastewater coming from milk processing contain nitrogenous, as well as domestic wastewater, but with sensible differences of concentration and degree of decomposition.
Main parameters characterizing dairy wastewater are:
- BOD: 800 – 2300 mg/l
- COD: 1700 – 3400 mg/l
- N tot: 31 – 53 mg/l
- Ammoniacal nitrogen.: 6,7 – 8,4 mg/l
- Nitrate nitrogen: 8,8 – 35 mg/l
- Phosphorous P tot.: 12,7 – 18,5 mg/l
- Suspended solids 525-550 mg/l
- pH: 5,3 -7,8
System usually requires a primary treatment as a primary sedimentation.
Plastic filters have achieved good results as pre-treatment even with BOD removal yields up to 60%, but flow shall not exceed an organic daily load of 2.3 kg BOD / m3 to prevent clogging.
Biological filtration (alternating double filtration) is a efficient method to achieve good effluent (about 20 mg/l BOD and 30 mg/l of suspended solids).
Usually it works with a hydraulic load of about 0.04 m3/m3 per hour and an organic load ranging between 0.19 and 0.28 kg BOD/m3 per day.
Effluent can be recirculated and mixed with inlet flow (downstream to pre-treatment) to achieve a better result.
The goal is to guarantees an incoming BOD concentration between 200 and 300 mg/l.
Biological filtration is preferred to activated sludge plant. The activated sludge is not recommended with high loads of BOD, due to bulking problems (swelling of activated sludge).
Neverless, with activated sludge plants it’s possible to get good results with careful handling, with inoculum of selected microorganisms and with a correct dosage of incoming flow.
The biologic reactor is designed according to domestic wastewater equivalence but with higher oxygen absorption and endogenous respiration parameters.
Well running extended aeration wastewater treatment plants, for dairy farming, can get good results even up to yields of 96%. In some cases, it can achieve a yield of 99% for BOD and 98% for suspended solids.
Moreover, the choice of proper air diffusers are very important. For example, submerged porous plate has most clogging problem than submerged turbine aerators or other mechanical system.
Trickling filters and activated sludge used together provide good results both for cleaning and resistance to load fluctuation.
Sludge volume index (SVI) can range between 12 up to 400 ml/gm due to sludge swelling.
Slight sludge chlorination reduces the phenomenon, in particular by dosing of aluminium polychloride and a polyelectrolyte.
Oxidation ditch is only used for small plants and low amounts of BOD.
Oxidation pond is another possible solution, but requires an additional oxygen supply and might have odour problems.
The anaerobic digestion gets very good results but it must always be followed by an aeration chamber to avoid unpleasant odour problems.
Aerobic filtration requires smaller retention times compared to anaerobic digestion (from 2 to 4 hours instead of 6 days), but, on the other hand, it has the problem of sulphur odours, periodic backwashing, a long start-up period and problems related to clogging caused by suspended solid.
The aerobic process is better for small plants, anaerobic for large systems: and it offers a good chance to improve energy by biogas recycling.
Chemical dosing, coagulation with sedimentation and aeration steps can acheive a good quality effluent.
Adsorption (activated carbon filter) or reverse osmosis and ultrafiltration are refining processes for very high quality effluent results.
Effluent quality varies greatly, depending on working cycle and dairy farm kind.
Small changes in dairy production can lead to significant water savings throughout the year. Due to high BOD concentration, less water means a great incoming BOD reduction.
Dairy wastewater recycling as agricultural fertilizer, is possible (according by local regulation).
Effluent has great fertilizing power but it contains a high sodium concentration.
In clayey soils, regulation usually sets not to exceed a concentration of 100 mg/l sodium.