Cheese dairy wastewater: why a grease trap alone is not enough
At small cheese dairies wastewater treatment often comes down to a single buried tank — a grease trap. After a few months of operation there is odour around the plant, cloudy water with a whitish film at the discharge point and complaints from neighbours. Let us look at why this happens and which stages are needed for dairy wastewater to be actually treated.
What cheese dairy wastewater contains
Dairy wastewater is not domestic sewage. It receives milk residues, whey, milk fat, proteins, lactose (milk sugar), cleaning and disinfecting solutions, and salt from cheese brining. The composition changes during the day: production sends one set of flows, equipment washing another.
The main load comes from organic matter dissolved in the water. It is measured by COD (chemical oxygen demand, a measure of all organic matter) and BOD (biochemical oxygen demand, a measure of the organic matter bacteria can oxidise). Combined dairy wastewater usually has a COD of 1,000 to 10,000 mg/l, domestic sewage a few hundred.
Whey is a separate matter. Each kilogram of cheese leaves about 8–9 litres of whey, and its COD is around 50,000–80,000 mg/l. Even a small amount of whey in the combined flow sharply raises the treatment load.
Two more features. The first is shock discharges: washing equipment produces in half an hour a volume comparable to hours of normal operation. The second is pH swings: acidic and alkaline cleaning solutions go to the drain in turn, and the pH of the wastewater can change from acidic to strongly alkaline within a shift.
What a grease trap does and does not do
A grease trap works by settling: free fat floats up, heavy particles sink. This is a useful first stage. It protects pipes and pumps from fat blockages and reduces the load on the following units.
But in dairy wastewater most of the fat is emulsified — broken into fine droplets that do not float on their own. Proteins are colloidal, and lactose is fully dissolved. Settling does not retain them. So after a grease trap COD falls only slightly, and wastewater that keeps decomposing goes to the discharge point.
Odour is the direct consequence. In a closed tank without oxygen, proteins and fats putrefy, releasing hydrogen sulphide and volatile fatty acids. If the grease trap is not cleaned on schedule, the accumulated layer of fat and sediment itself becomes a source of odour.
Treatment stages for dairy wastewater
A working scheme for a cheese dairy is assembled from several stages. Each solves its own task and does not replace its neighbour.
| Stage | What it retains | What it lets through |
|---|---|---|
| Screen and grease trap | coarse debris, free fat, sand | fat emulsion, proteins, lactose |
| Equalisation tank | evens out flow, pH and concentration | does not remove pollutants as such |
| Chemicals and DAF flotation unit | emulsified fat, proteins, fine suspended solids | dissolved lactose and part of the organics |
| Biological treatment | dissolved organics, reduces BOD and COD | excess sludge that has to be separated |
| Sludge dewatering | water from float sludge and biological sludge | filtrate is returned to the head of the scheme |
The equalisation tank is a mixed tank into which all flows drain. It smooths out shock discharges after washing and mixes acidic and alkaline solutions, so pH partly evens out by itself. The following stages receive a flow with predictable rate and composition. If equalisation is not enough, a pH sensor is installed in the tank and acid or alkali is dosed. Wastewater should not be kept in the equalisation tank for more than a day: without aeration it turns sour and smells, so the tank is mixed and aerated if necessary.
A DAF flotation unit (dissolved air flotation) saturates part of the water with air under pressure. When the pressure is released, microbubbles form, attach to pollutant flocs and lift them to the surface, where a scraper removes the float sludge. A coagulant and a flocculant are dosed before the unit: the coagulant breaks the emulsion and gathers fine particles, the flocculant enlarges the flocs. For dairy wastewater the choice of chemicals and pH is especially important — proteins and detergents strongly affect the result. How a flotation unit is built and sized is described in the article on DAF flotation.
Biological treatment is needed because lactose and dissolved organics pass through flotation. They are oxidised by activated sludge bacteria in an aerated reactor. For plants with uneven flow an SBR is often used — a sequencing batch reactor in which filling, aeration, settling and decanting take place in turn in one tank. One such scheme is SOLVIA PneumoSBR.
Why biology cannot go without pretreatment
Sometimes, to save money, wastewater is sent from the grease trap straight into the bioreactor. The result is predictable. Fat coats the sludge flocs and prevents them from getting oxygen. A sharp load increase after a whey discharge eats up the oxygen in the reactor. pH swings from cleaning solutions inhibit the bacteria. Foam, odour and poorly settling sludge appear, and the sludge is carried out with the treated water.
That is why the sequence matters: equalisation and flotation first remove fat, proteins and load peaks, and then biology works with wastewater it is adapted to.
Whey and sludge: decide before the design
The most effective way to reduce the treatment load is to keep whey out of the drain. It is collected separately and sent for processing, for animal feed or for separate treatment. The difference in the size and cost of the treatment plant can be several-fold. The decision on what to do with whey is made before the plant is sized, because it changes the input data.
The second question that is often postponed is sludge. The flotation unit produces fatty float sludge, biology produces excess sludge. If they are simply stored, they putrefy and smell. Usually the sludge is dewatered on a screw or belt press and taken away. Which press suits is covered in the article on sludge dewatering.
Requirements for effluent quality at the discharge point are set by the water utility or the local regulator, and they differ for each site. Which parameters are monitored is described in the article on discharge control.
What next
To choose a scheme, gather data on production: how much milk you process per day, which products you make, where the whey goes now, how much water goes to the drain per shift and in the peak washing hour. Have the wastewater analysed at the outlet from the workshop: COD, BOD, fats, suspended solids, pH. Take samples at different times of the shift rather than a single one: dairy wastewater composition varies a lot.
Add a description of the existing system, photos of the tanks, the dimensions of the area for equipment and the discharge point. With this data the equalisation tank can be sized and the capacity of the flotation unit and the volume of the biological stage selected. Flotation units are collected in the section DAF flotation units, complete lines in the section industrial wastewater treatment.