Evolution of organic matter and nitrogen during co-composting of olive mill wastewater with solid organic wastes

Four olive mill wastewater (OMW) composts, prepared with three N-rich organ ic wastes and two different bulking agents, were studied in a pilot plant u sing the Rutgers system. Organic matter (OM) losses during composting follo wed a first-order kinetic equation in all the piles, the slowest being the OM mineralisation rate in the pile using maize straw (MS). The highest N lo sses through NH, volatilisation occurred in the mixtures which had a low in itial CM ratio and high pH values during the process. Such losses were redu ced considerably when MS was used as the bulking agent instead of cotton wa ste (CW). N fixation activity increased during the bio-oxidative phase befo re falling during maturation. This N fixation capacity was higher in piles with a lower NH4+-N concentration. Only the composts prepared with OMW, CW and poultry manure or sewage sludge reached water-soluble organic C (C-W) a nd NH4+-N concentrations and C-W/N-org and NH4+/NO3- ratios within the esta blished limits which indicate a good degree of compost maturity. Increases in the cation-exchange capacity, the percentage of humic acid-like C and th e polymerisation ratio revealed that the OM had been humified during compos ting. The germination index indicated the reduction of phytotoxicity during composting.