Comparison of techniques to assess compost maturity and stability

A number of physiochemical and microbiological stability parameters of compost were compared and their suitability in monitoring the composting process were evaluated. Two composting piles were constructed as open-windrow systems under local weather condition. A farm manure waste windrow (MA) was made from fresh feed-pad dairy manure and straw, spoiled hay, wood chips and poultry litter, while a green waste windrow (GW) consisted of municipally collected green waste. The changes observed in selected parameters, included the contents of C, N, organic matter (OM) and water soluble C (WSC), 13C NMR spectroscopy, thermogravimetric analysis, microbial respiration, FDA hydrolysis activity, microbial biomass and nitrate and ammonium content, were investigated in samples collected from the four different phases of composting. The results showed OM transformations occurring during the composting process of both GW and MA windrows can be revealed from 13C NMR spectroscopy and thermogravimetric analysis. In addition, this study again confirmed that WSC can act as a reliable measurement of the stability of compost regardless of the type of initial materials. The significant loss of OM (33.8% in MA windrow and 25.0% in GW windrow) and low values of WSC (2.46 and 2.02 mg per g of dried sample in MA and GW compost respectively after 175 days of composting) from both windrows were in agreement with the molecular, microbiological parameters and thermal analysis and confirmed that the end products from both windrows were fully stabilized. In light of the results obtained from the biochemical parameters, the reliability of using chemical strip tests for monitoring stability of compost on farm were evaluated by correlating the method results with standard chemical tests and validated by comparing with other stability parameters. The content of NO3--N and NH3-N were estimated by the proposed Strip Test Methods and the methods were found to be strongly correlated with the standard chemical analysis (r= 0.967 & 0.977 for NO3--N and NH3-N respectively, p<0.01). In addition, an NH3 strip test was found to have high r2 value with WSC in both the GW (0.97, p<0.01) and MA windrows (0.93, p<0.01) indicating that both WSC and NH3 strip test can be used as maturity indicators regardless of the substrate mixtures used in this study.