Physico-chemical and microbial evaluation of low technology methods for composting urban market crop waste
Tumuhairwe, John Baptist
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Composting has been practiced globally for management of organic urban solid wastes. In the urban areas of Uganda, composting has not been actively practiced yet there is an increasing burden of wastes with composition of over 70% biodegradable (about 1000 t per day). The process is mediated by a consortium of fungi and bacteria in succession to achieve mature and high quality compost. Therefore, knowledge of the population, ecology, diversity and abundance of these microorganisms is essential in accelerating the composting process. It is also important to identify and determine the succession of these microorganisms to enhance the composting process. During composting, the wastes undergo physical and chemical transformations before the compost is mature and ready for use. It is important to know and understand these transformations because they determine the quality, quantity and stability of compost. Since composting has not been adopted as an urban waste management technology in Uganda, this study explored urban market crop waste (MCW) composting using four low-technology methods which would be applicable on a subsistence scale by smallholder farmers. The methods used were above ground covered (AC), above ground open (AO), pit covered (PC) and pit open (PO). These methods were compared for their effects on accelerating compost maturity. The effect of these methods on the population, diversity, abundance and succession of bacteria and fungi during composting was also determined. The physical and chemical transformations were monitored through determination of changes in pH, temperature, total organic carbon (TOC), total nitrogen (TN), organic nitrogen (org N), total potassium (TK) and total phosphorus (TP). Other compost process indicators measured were water-soluble carbon (WSC), ammonium-N (NH4 +-N) and nitrate-N (NO3 --N). Compost maturity process indicator ratios used were C/N, NH4 +-N/ NO3 --N, WSC/ organic nitrogen and WSC/ total nitrogen. Polymerase chain reaction (PCR) followed by terminal restriction fragment length (T-RFLP) and pyrosequencing methods of 16S rDNA and 18S rDNA were used to identify and determine bacterial and fungal succession, respectively, during composting. Temperature in the composting wastes rapidly increased reaching over 60oC within 7 days and later gradually reduced as compost progressed to curing phase and maturity. Similarly, pH increased reaching highest alkalinity pH of about 9.0 within the 21 days and slightly declined but remained alkaline. The concentration of TOC, WSC, TN, TP, TK and NH4 +-N significantly decreased (p<0.05) but increased for NO3 --N, as composting progressed to maturity. All four composting methods produced mature composts within 63 days. Suitable maturity indicators for MCW compost were C/N ratio <12, WSC <1%, NH4 +-N <400 mg kg-1, NH4 +-N/NO3 --N <0.2, WSC/TN, WSC/organic- N <1. On the basis of maturity indicators, the AC method generally enhanced composting than the AO, PC and PO methods but pit methods require much less investment costs and are more suitable for smallholder farmers. The T-RFLP and pyrosequencing molecular techniques showed high population diversity and abundance of bacteria than fungi in compost from tropical urban market crop waste compost. Principal component analysis (PCA) of the terminal restriction fragments (TRFs) showed no significant differences among the four composting methods for bothfungi and bacteria. Pyrosequencing of 16S rDNA generated over 110,000 bacteria sequences of about 230 bp in length. Using these sequences, rarefaction, Chao 1, cluster analysis at 3% dissimilarity showed that bacteria community diversity declined between early thermophilic and matured compost in all methods. The Shannon index (H') showed a decreasing diversity and increasing evenness as compost progressed from the thermophilic phase to maturity for all composting methods. This pattern of community mobility is attributed to selection of bacteria as affected by changes in temperature, pH and available nutrients. The most abundant phylotypes were Proteobacteria, Firmicutes, Bacteriodetes and Actinobacteria. Pyrosequencing provided more information on compost bacterial community diversity and abundance than previously used molecular methods. Plant growth promoting bacteria as well as some human and plant pathogen bacteria were detected during composting. However, it was not clear whether the DNA was from dead or active microorganisms. Pyrosequencing of 18S rDNA was not as successful as for 16S rDNA. Pyrosequencing of the 18S rDNA region between ITS1 and ITS4 on 454 GS FLX platform generated few sequences to accurately estimate fungal population, diversity and abundance during composting of urban market crop waste. The detected common fungal species in the compost were Scedosporium, Candida, Trichosporon and Pseudallescheria. There were no phytopathogenic fungi detected, implying that compost from all the methods used was safe for soil fertility management.