RO Membrane Filtration of Pretreated Swine Manure with High Levels of Suspended Solids

Abstract

Sustainable practices require that manure nutrients be reused within the agricultural sector. Reverse osmosis (RO) is one of the few technologies capable of producing nitrogen and potassium concentrates that can be transported to fertilize distant cultivated fields. However, solid-liquid separators produce a liquid fraction that could significantly foul RO membranes because of high colloidal and particulate concentrations. This project investigated the effect of high suspended solids (SS) concentrations, ranging from 258 to 3897 mg L-1, in pretreated manure on the performance of a spiral-wound RO membrane. The effluents were first concentrated to 47% of the initial volume. The concentrates, containing from 453 to 6893 mg SS L-1, were then filtered in full-recycle mode for a total of 22.5 h. Permeate flux stabilized during the full-recycle filtration period at SS levels up to 4000 mg L-1 in RO concentrate, but it was 13% lower at 4016 than at 453 mg SS L-1. At the highest SS level of 6893 mg L-1 in concentrate, flux decreased throughout the full-recycle filtration period, and it was 27% below that measured at 453 mg L-1 after 22.5 h. Lower flux was attributed to the formation of a concentrated gel layer at the membrane surface. While the thickness or density of the gel layer appeared to stabilize below 4000 mg SS L-1 in concentrate, it kept increasing at the highest tested SS level. However, with all effluents, the gel layer was easily removed by rinsing the membrane with tap water, and water flux recovery after rinsing (82.3% 1.6%) and chemical cleaning (99.1% 1.2%) were similar at all SS concentrations. Results thus suggested that spiral-would RO membrane systems could be used to directly process swine manure pretreated by physically based solid-liquid separators, but permeate flux will be significantly reduced at SS concentrations above 4000 mg L-1 in the membrane element.