Co-composting of horticultural waste with fruit peels, food waste, and soybean residues.

Horticultural waste was co-composted with fruit peels, food waste, and soybean residues individually to evaluate the effects of these easily available organic wastes in Singapore on the composting process and product quality. Each co-composting material was mixed with horticultural waste in the wet weight ratio of 1:1 and composted for 46 days. Results showed that all co-composting materials accelerated the degradation of total carbon and resulted in higher nutrients of nitrogen (N), phosphorous (P), and potassium (K) in the final product compared with horticultural waste alone. Mixture with fruit peels achieved the fastest total carbon loss; however, did not reach the minimum required temperature for pathogen destruction. The end product was found to be the best source for K and had a higher pH that could be used for the remediation of acidic soil. Food waste resulted in the highest available nitrate (NO3-N) content in the end product, but caused high salt content, total coliforms, and slower total carbon loss initially. Soybean residues were found to be the best co-composting material to produce compost with high N, P, and K when compared with other materials due to the highest temperature, fastest total carbon loss, fastest reduction in C/N ratio, and best conservation of nutrients.

Evaluation of Lighting Systems, Carbon Sources, and Bacteria Cultures on Photofermentative Hydrogen Production.

Fluorescent and incandescent lighting systems were applied for batch photofermentative hydrogen production by four purple non-sulfur photosynthetic bacteria (PNSB). The hydrogen production efficiency of Rhodopseudomonas palustris, Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodospirillum rubrum was evaluated using different carbon sources (acetate, butyrate, lactate, and malate). Incandescent light was found to be more effective for bacteria cell growth and hydrogen production. It was observed that PNSB followed substrate selection criteria for hydrogen production. Only R. palustris was able to produce hydrogen using most carbon sources. Cell density was almost constant, but cell growth rate and hydrogen production were significantly varied under the different lighting systems. The kinetics study suggested that initial substrate concentration had a positive correlation with lag phase duration. Among the PNSB, R. palustris grew faster and had higher hydrogen yields of 1.58, 4.92, and 2.57 mol H2/mol using acetate, butyrate, and lactate, respectively. In the integrative approach with dark fermentation effluents rich in organic acids, R. palustris should be enriched in the phototrophic microbial consortium of the continuous hydrogen production system.