The following article will guide you about how to treat sewage from dairy industry.
Sewage from Dairy Industry:
The volume of sewage from different dairies may vary from 6 to 10 litres per litre of milk processed depending on the processes used, products made, care taken in use of water and the quantity of water available. The various process involved in dairies include pasteurization of milk and bottling of pasteurized milk, and production of various milk products such as butter, ghee, cheese, chenna, condensed milk, powdered milk, baby food and ice-cream.
The sewage from dairies includes wash water from milk cans, tanks or drums, bottles, equipment, pipe line and floor. It also includes portions of spilled milk, spoiled or sour milk, skimmed milk, whey and butter milk. The nature and composition of sewage from a dairy depend on the quantity of milk processed and the type of products made.
Since sewage from a dairy consists primarily of dilutions of milk and its products, it has a high BOD value. Under Indian conditions a BOD value of 1000 mg/l can be considered as a usual average value. Alternatively the BOD load for sewage from dairies may be considered as 2 to 8 kg per cubic metre of milk processed.
Further suspended solids form a good portion of the total solids in the dairy sewage and oil and grease are relatively high. Volatile solids constitute about 70 to 90 per cent of the total solids. The BOD to COD ratio of sewage from dairies is in the range of 0.625 to 0.71 indicating their easy biodegradability.
Disposal of Dairy Sewage:
The dairy sewage may be disposed of by discharging either into inland surface waters, or on land, or into municipal sewers.
However, discharge of untreated dairy sewage in each of these cases has pollutional effects as indicated below:
(1) Discharge into Inland Surface Waters:
The following are the pollutional effects if untreated dairy sewage is discharged into inland surface waters:
(i) Depletion of Dissolved Oxygen:
The most important problem of water pollution associated with dairy sewage is the depletion of dissolved oxygen. As dairy sewage contains all the nutrients essential for bacterial life and as the water temperatures encountered in most parts of our country are ideal for bacterial growth, the rate of decomposition is high resulting in anaerobic conditions followed by bad odours, conditions suitable for fly and mosquito breeding.
When discharge into open ‘Nallah’ and ponds the following adverse conditions are likely to arise:
(a) Putrefaction due to rapid degradation of lactose and production of lactic acid and butyric acid leading to foul odours.
(b) Fly breeding on the sludge rafts floated by anaerobic decomposition, and
(c) Grease floating on the top and adhering to aquatic weeds leading to ugly sight.
(ii) Effect on Fish:
It has been found that at a dilution of 1 : 15, dairy sewage (excluding whey) has distressing effects on fish. Whey is found to become toxic to fish in a few hours even at a dilution of 1 : 35. Lactic acid is toxic to fish at a concentration of 654 mg/l in hard water or even less in soft waters and when the dissolved oxygen content is low.
Dairy sewage contains soaps or detergents that are toxic to fish at concentrations of 600 mg/l. Butyric acid, a product of anaerobic metabolism, has a very unpleasant rancid odour. The presence of butyric acid with concentration beyond 100 mg/l adversely affects and above 400 mg/l kills trout fish. It also affects Daphnia (a crustacean) and Scenedesmus (an algae).
Dairy sewage can occasionally carry pathogenic bacteria.
Milk may sometimes contain strontium 90, a beta-emitting radio-isotope when pasture land is contaminated with this isotope due to radioactive spills or fallout of fission products. Thus dairy sewage may also contain such radio-isotopes.
(2) Discharge on Land:
Disposal of dairy sewage on land for irrigation is one of the most commonly practised methods in our country. However, all soils are not suitable for this purpose. Heavy soils like clay can lead to ponding, anaerobic conditions and foul odours if the irrigation system is not properly designed.
(3) Discharge into Municipal Sewers:
As dairy sewage is readily biodegradable it is often presumed that there is no problem in treating it along with domestic sewage. However, dairy sewage contains significant amount of carbohydrate and the temperature of discharge is ideal, there is great possibility of dairy and domestic sewage mixture becoming septic in the sewer itself, which may need additional pre-aeration in the sewage treatment plant.
Because of the pollutional effects of the untreated dairy sewage, it needs to be treated prior to its disposal.
The methods of treatment of dairy sewage are indicated below:
The various methods recommended for the treatment of dairy sewage are as follows:
(a) Preliminary treatment,
(b) Primary treatment, and
(c) Secondary treatment.
(a) Preliminary Treatment:
This involves removal of coarse solids and oil and grease for which screens and grease traps are invariably used. Further it is necessary to provide for equalization of flows and aeration to avoid production of odour and septic conditions.
(b) Primary Treatment:
Primary treatment such as settling and digesting the sludge is not very useful for dairy sewage. However, dilution with water and anaerobic lagooning or aerated lagooning for a short period can reduce the BOD and suspended solids enough to facilitate disposal into municipal sewers or on land for irrigation.
The dairy sewage is likely to have BOD about 1000 mg/l and suspended solids below 1200 mg/l, and to reduce both BOD and suspended solids to 50 per cent to suit the limits for its disposal into municipal sewers or on land for irrigation, the dilution water required is approximately in the ratio of 1:1.
The BOD and suspended solids can also be reduced to 50 per cent by treating in anaerobic lagoons with detention time 6 days or in aerated lagoons with detention time 1.5 days. The dilution requirement and the detention periods required in the lagoons may have to be modified depending on the characteristics of the dairy sewage. Fig. 19.1 shows a flow diagram indicating the above mentioned alternative methods of treatment of dairy sewage for its disposal into municipal sewers or on land for irrigation.
(c) Secondary Treatment:
Secondary treatment is required for further reduction of BOD to suit the standards for the disposal of the dairy sewage into inland surface waters.
The secondary treatment may consist of one of the following methods:
(i) Oxidation ditch
(ii) Aerated lagoon; and
(iii) Anaerobic lagoon followed by oxidation pond
(i) Oxidation ditch is preferred because of its compact nature and simplicity in construction and operation. The oxidation ditch can be loaded at 0.2 kg BOD/kg MLSS/day. The BOD reduction that can be expected would be 95 to 98 per cent.
(ii) The aerated lagoon is designed to have a detention time of 3.7 days to get a 90 per cent BOD reduction.
(iii) Anaerobic lagoon treatment for 10 days would help reduce the BOD by 90 per cent. It is desirable to reduce the residual BOD by aerated lagoons or stabilization ponds or by dilution with water. One of the above methods would help to ‘sweeten’ the sewage, that is, to reduce the septicity or increase the oxidation reduction potential.
Fig. 19.2 shows a flow diagram indicating the above mentioned alternative methods of treatment of dairy sewage to be discharged into inland surface waters.