Effect of bulking materials and mixing ratios on concentration of nutrients during composting of raw faecal sludge from peri-urban areas

The objectives of this study were to (1) examine the suitability of two common bulking materials (BMs) used in composting faecal sludge (FS) and (2) investigate the effect of BM and mixing ratio on raw FS compost mixture on selected compost parameters.

Scientists and researchers over the world are exploring many options to address the FS management problems that confront our environment. Large quantities of FS are generated daily and these are not properly disposed of due to lack of adequate treatment and disposal facilities. This however, need to be suitably treated and (or) properly disposed of in order to reduce their environmental impact. Composting is a widely used cost-effective and environmentally acceptable method for treating solid or semisolid biodegradable waste (Malińska & Zabochnicka-Świa˛tek 2013). Composting is the biological decomposition of biodegradable waste which involves a complex transformation of the raw organic substrates, which are degraded and transformed into stable organic matter. During the biodegradation of the compost feedstock, nutrients such as nitrogen, phosphorus and potassium are released and recycled in various chemical forms through the microorganisms and invertebrates that make up the compost food web and become available for uptake by plants or microorganisms. These by-products are considered a critical factor for agricultural production (Senesi 1989; Haider 1994; Chen et al. 1996). There are two fundamental types of composting and these are aerobic and anaerobic (Tweib et al. 2012). These two types of biological treatments are directly related to the type of bacteria or microorganisms that are involved in the degradation of organic impurities in a given sludge and the operating conditions of the bioreactor. The aerobic treatment processes in the presence of air utilize those microorganisms (also called aerobes), which use molecular/free oxygen to assimilate organic impurities that is convert them in to carbon dioxide, water and biomass (Mittal 2011). Anaerobic method involves a series of process in which microorganisms break down biodegradable material in the absence of oxygen.

Composting of raw FS poses many difficulties due to its high MC, high wet bulk density and low carbon to nitrogen (C/N) ratio, thus making it susceptible to compaction (Malińska & Zabochnicka-Świa˛tek 2013). High MC and lack of structure cause additional compressive stress and compaction on a compost bed resulting in poor air-filled porosity and permeability through a composting pile (Malińska & Richard 2006). A study by Chang & Chen (2010) reported that the composting process may be more effective when the C/N ratio and the MC is managed according to material for the compost. BM sometimes referred to as bulking agents play a very important and effective role in composting by controlling the air supply, C/N ratio, moisture and other important composting parameters. Inasmuch as BM are necessary for composting, improper type and insufficient amounts (ratio) in a composting matrix could cause poor aeration and moisture transfer resulting in the release of odorous gases and production of poor compost quality.

Literature on FS to BM ratios during composting process is diverse. The ratio of FS and BM is an important consideration in optimizing the composting process (CWMI 2004). The quality of compost is also largely influenced by the proportions of sludge and BM that is used. Oleszkiewicz & Mavinic (2001) recommended that BM for composting should be 40–50% TS. Banegas et al. (2006), in a study on composting anaerobic and aerobic sewage sludges using two proportions of sawdust suggested mixing ratios of 1:1 and 1:3 sludge:sawdust (v:v). For the aerobic composting, sludge was subjected to a process of aerobic composting with periodic turning. Molla et al. (2004) used a 1:1 (w:w) ratio in a study to evaluate the feasibility of the solid bioconversion processes in the biodegradation of wastewater sludge. Bousselhaj et al. (2004), in studying the nitrogen fertilizer value of sewage sludge co-compost, mixed sewage sludge with different BM (domestic solid waste, olive cakes and sawdust) using also a 1:1 (w:w) ratio. Eftoda & Mc Cartney (2004) employed wood chips as BM, assaying the ratios of 1:1, 1:2, 1:3 and 1:4. Hay et al. (1988), composted sewage sludge with either straw or sawdust in a sludge:BM ratio of 1:2 and 1:1 (v:v), respectively.

Due to complexity of composting parameters, requirements and processing costs, it is of utmost importance to determine the type and most optimal ratio of the BM in a composting mixture (Malińska & Zabochnicka-Świa˛tek 2013).

Raw FS and BM (wood chips and maize cobs) were sampled from three peri-urban communities in the Ashanti region of Ghana for this study. The raw FS and BM were transported to the Kwame Nkrumah University of Science and Technology for composting.

Cylindrical plastic reactors were used for the composting experiment. Each reactor was sized 27 litres measuring 32 cm in diameter and 33 cm in length. A hole of 5 inches was drilled at about 10 cm from the bottom of the reactor to provide an inlet for cold air expected to flow by convection into the composting mixture and, once warmed, to exit through (Adhikari et al. 2009) another 5 inches hole on the top opening which is about 5 cm from the top end of the reactor.

Compost feed stocks were prepared by manually mixing the raw FS and BM using a shovel in volume by volume ratio; 1 part of FS:1 part of BM, 1 part of FS:2 part of BM and 2 part of FS:1 part of BM; Wood (2:1), Wood (1:1), Wood (2:1) and Maize cob (2:1) Maize cob (1:1) and Maize cob (1:2). A control of only FS was also monitored. Turnings were performed once every three days during the first 14 days of active composting to ensure the restructuring and maintain a uniform decomposition of the compost. Turnings were later performed at regular intervals once every week till the end of the compost period which lasted for 60 days. Seven (7) experimental trials each with replication (14 experimental trials), were composted and analysed as shown in Table 1.

Total carbon (TC) and nitrogen, C/N ratio, phosphorus and potassium were measured in compost. Grab samples weighing 10 g were randomly collected from three locations (upper, central and lower) within the plastic reactor. All the grab samples were mixed together to obtain homogenised samples which were immediate analysed following standard methods for the examination of water and wastewater (APHA-AWWA-WEF 2001).

The materials used in the composting process comprised raw FS, wood chips and maize cobs (chopped). The wood chips are by products of wood processing whereas maize cobs are obtained after removal of the kernels. These materials provide the free air space, moisture control and maintain the C/N ratio during composting (Adhikari et al. 2008). The general characteristics of the BM and raw FS were initially monitored before they were composted and the results are presented in the Table 2.

Carbon content was higher in Wood chips (53.6 ± 0.30) compared to maize cobs (49 ± 2.0). Nitrogen and Phosphorus levels in Maize cobs were however, greater than that present in the wood chips (Table 2). The measured C/N ratio for the raw FS was very low and was not suitable for composting. Also, the BM used for the composting process showed an initial low MC and high C/N ratio (Table 2). This is indicative of the fact that composting of raw FS requires addition BMs that allow optimal moisture, C/N ratio and also provide structural support to obtain sufficient air-filled porosity to improve on the compost process.

The mean values (n = 2) of measured compost parameters at the initial and final stages of the compost process are presented in Table 3. Also, average values on weekly basis were determined and presented in their respective figures.

The C/N ratio is widely used as an indicator of compost maturation and should become stable with time. The final C/N ratios of approximately 10 to 20 for all the experimental trials indicated that the compost were matured and they would be good sources of nitrogen. It can generally be concluded that wood chip and maize cob can be considered a good BM for use with FSs. All the mixing ratios assayed allowed composting to develop adequately compared to the control. However, the experimental trial, Maize cob (1:2) is preferred to Wood (2:1), Wood (1:1), Wood (1:2), Maize cob (2:1) and Maize cob (1:1) due to the fact that it contained more NPK and also C which are essential nutrients for plant growth and for improving on the soil organic matter content, respectively, hence increasing its soil fertility improvement value. Although the final compost materials appear to be suitable for agronomic use, the levels of the three major nutrient elements (N, P, and K) of the finished product of compost are not high enough compared to inorganic fertilizer. The compost can also be used as soil conditioner or as an environmentally friendly alternative to solve the disposal problems of FS menace.

This work was financed by the Sani UP Project supported by Bill and Melinda Gates Foundation. The authors would like to thank Kingsley Osei Bonsu of the Environmental Engineering Quality Laboratory of Kwame Nkrumah University of Science and Technology.