Dairy effluent treatment

The first step before considering treating the pollution generated by a production unit in the dairy industry is to carry out a pollution assessment to study the sources of pollution, characterize discharges and try to minimize them.

The first stage in wastewater treatment involves the separation of easily recoverable forms of pollution (particles, for example). The aim is to minimize the amount of pollution to be treated in the subsequent treatment process. Dairy industry effluents can therefore undergo various pre-treatments: screening, degreasing and grit removal, with the aim of eliminating particulate pollution.

Dairy effluent, which contains mainly organic pollution in soluble form, can then be spread on agricultural land or treated biologically. In the former case, the pollution is eliminated by soil micro-organisms and plants. In the second case, microbial populations present in biological reactors are entrusted with the task of eliminating the pollution.

There are two main strategies for deploying micro-organisms: aerobic and anaerobic.

The aerobic route uses micro-organisms contained in biological reactors in which oxygen is transferred either naturally or artificially, by means of surface turbines or bottom diffusers.

The oxygen and soluble organic or mineral nutrients (carbon, nitrogen, phosphorus...) that make up the pollution will serve as substrates for the micro-organisms that multiply. A fraction of the soluble pollution is eliminated in gaseous form (CO ₂ for example) and the complementary fraction is transformed into insoluble pollution (the micro-organisms). It can be recovered by decantation and forms the sludge.

The techniques used include aerated lagoons, bioreactors applying the activated sludge technique, bacterial filters, sequencing batch reactor (SBR), etc.

This approach consumes energy to transfer oxygen and produces sludge. Between 0.3 and 0.5 kg of sludge (expressed as dry matter) can be produced per kg of chemical oxygen demand (COD) removed, depending on process characteristics.

This technique can produce treated water of a quality compatible with discharge into the natural environment. The management of the resulting sludge often remains a problem. Activated sludge loads range from 0.5 kg of COD per cubic meter of reactor per day for low-load systems to 10 kg of COD per cubic meter of reactor per day for high-load systems.

In anaerobic digestion, polluting organic matter is transformed by micro-organisms in the total absence of...