Membrane Separation of Raw and Anaerobically Digested Pig Manure

Abstract

Manure ought to be controlled and handled environmentally soundly. Volume reduction of surplus manure by reverse osmosis (RO) is a viable and economic option in certain areas where another alternative is to reduce the number of animals. This membrane separation project arose out of an urgent need to re-establish an ecological balance between farming and the natural environment without necessarily having to decrease the number of livestock. The suggested technical solutions should not significantly reduce the standard of living of the farmers. Dewatering of manure makes it easier to store and transport the nutrients. RO coupled to anaerobic digestion as pretreatment with inherent production of biogas energy leads to on-site production and use of biological fertilizer and humus. Such treatment will improve the hygienic conditions and general health of the farming communities compared to purchase and sole use of chemical fertilizers. Mechanical dewatering of raw as well as anaerobically digested pig manure yields a cake of 25 % solids and a liquid fraction containing 1 % solids. The solid cake contained virtually all the phosphorous, whereas nitrogen was dissolved equally in water in both fractions. A tubular RO membrane plant separated 95 % of the total nitrogen in the liquid fraction. The particle free permeate met the effluent criteria for direct discharge to the neighbouring fjord. The concentrate approached one tenth of the original manure volume and is considered a liquid fertilizer. Cleaning of the membranes was performed with nitric acid and the alkaline detergent Ultrasil 11. The membrane flux was fully recovered after each chemical cleaning.