The treatment and disposal of milk factory effluents I By means of percolating filters and septic tanks

Reviews previous investigations of methods of purifying milk factory effluents and describes laboratory experiments carried out as part of the programme of the Water Pollution Research Board, Department of Scientific and Industrial Research, on the biological treatment of these effluents on percolating filters and in septic tanks. The same sectional filters were used as in experiments on the biological filtration of dilute sucrose solutions. The filter medium was washed gravel passing 1-in. mesh but retained by 1/2-in. mesh. Tests were also made in which the top section of one of the filters was replaced by a section of wooden laths. The septic tank was a rectangular glass tank of 15 litres capacity. The experiments showed that mixtures containing 1-3 per cent, of fresh milk in water possess the requisite organic and inorganic substances for bacterial growth and in this respect are suitable for treatment by biological oxidation in percolating filters. Washing soda, which is used as a cleanser in dairies and milk factories, appears to delay the growth of the biological film in the first sections of the filter but to have little or no effect on the lower sections. The mechanical separation and accumulation of fat in the filter inhibits its own oxidation and causes clogging, resulting in a low rate of purification. This effect appears to be associated with an abnormal growth of fungi. Preliminary treatment in a septic tank brings about separation of fat and thus avoids this difficulty in subsequent filtration. A rapid lactic acid fermentation also occurs, the effect of which depends on the buffer capacity of the salts in solution. With distilled water containing 1 per cent, of milk the reaction of the liquid falls below pH 4-6 (the iso-electric point of casein) and causes precipitation of casein and inhibition of proteolysis. When hard tap water is used the pH of the liquid may not fall below 5-5; the casein then remains in solution and undergoes digestion by proteolysis. Even under very acid conditions the separated liquid is suitable for treatment on a percolating filter at a rate of 100 gal. per cu. yd. per day. A sludge of low nitrogen content remains. The use of alkali in the wash waters also favours proteolysis in the storage tank. The addition of waste waters containing sugar, however, results in abnormally high acidities and may cause precipitation of casein even in solutions of high buffer capacity. The resulting effluents, especially if of high B.O.D., have wide carbon/ nitrogen ratios and would not be particularly suitable for treatment on a percolating filter without addition of available nitrogen and probably also of phosphates. Addition of increased quantities of soda to induce proteolysis would appear to be the best way of overcoming the difficulties. Both the fermentation and the separation of fat are retarded at temperatures below 10°C, Laboratory and large-scale experiments are now being made on an alternative method of overcoming the difficulty of ponding which occurs when waste washing waters containing fresh milk are treated directly on a percolating filter. Two filters are used in series, the order being reversed when some solid matter has accumulated in the first.