The biodegradability of water hyacinth for biogas and biofertilizer production was studied under mesophilic conditions. The effects of water hyacinth pretreatments were also included in this investigation. It was found that water hyacinth has a low biodegradability of 27% when monodigested, while in a 3:1 ratio with cow manure, the biodegradability increases to 46%. At this elevated biodegradability, the water hyacinth biomethane potential was 185 LCH₄/kgVS, while that of cow manure was 216 LCH₄/kgVS. The Gompertz kinetic model had superior parameters than the logistic model for most of the water hyacinth–cow manure combined substrate digestion. Based on the Gompertz model, the lag phase and daily maximum methane production rate were 5.5 days and 22.9 mL/day, respectively, for the 3:1 codigestion (R² of 0.99). These values were 6.7 days and 15.2 mL/day, respectively, in the case of water hyacinth monodigestion (R² = 0.996). The dominant microbial species detected in the digestates were Bacteroidetes and Proteobacteria. A few microbial species were indigenous to water hyacinth, but more diverse consortia, which are key to efficient substrate biodegradation, came from cow manure. The digestate contained ammonium nitrogen at 68 mg/kg with phosphorous and potassium at 73 and 424 mg/kg, respectively. Nitrogen was lower but phosphorous and potassium were comparable to previously studied digestates of other substrates. Only water hyacinth pretreated by aerobic composting was proven to unlock a higher methane yield that matched the 3:1 codigestion with cow manure. Other pretreatments induced better biodegradation performance than that observed in untreated water hyacinth but these improvements were not as good as that of the 3:1 codigestion scheme. It was concluded that water hyacinth sourced from the Hartbeespoort Dam could be treated by anaerobic digestion to recover biogas and biofertilizer. However, more experiments are required to fully understand and harness the optimisation opportunities available in applying this technology to manage water hyacinths.

The biodegradability of water hyacinth for biogas and biofertiliser production was studied under mesophilic conditions. The effects of water hyacinth pre-treatments were also included in this investigation. It was found that water hyacinth has a low biodegradability of 27% when monodigested while at a 3:1 ratio with cow manure, the biodegradability increases to 46%. At this elevated biodegradability, the water hyacinth biomethane potential was 185 LCH4/kgVS, while that of cow manure was 216 LCH4/kgVS. The Gompertz kinetic model had superior parameters than the Logistic model for most of the water hyacinth-cow manure combined substrate digestion. Based on the Gompertz model, the lag phase and daily maximum methane production rate were 5.5 days and 22.9 ml /day respectively for the 3:1 codigestion (R2 of 0.99). These values were 6.7 days and 15.2 ml/day respectively, in the case of water hyacinth monodigestion (R2 = 0.996). The dominant microbial species detected in the digestates were Bacteroidetes and Proteobacteria. A few microbial species were indigenous to water hyacinth but a more diverse consortia which is key to efficient substrate biodegradation, came from cow manure. The digestate contained ammonium nitrogen at 68 mg/kg with phosphorous and potassium at 73 and 424 mg/kg respectively. Nitrogen was lower but phosphorous and pottasium were comparable to previously studied digestates of other substrates. Only water hyacinth pretreated through aerobic composting proved to unlock a higher methane yield that matched a 3:1 codigestion with cow manure. Other pretreatments induced better biodegradation performance than that observed on untreated water hyacinth but this improvements were not as good as that of the 3:1 codigestion scheme. It was concluded that water hyacinth sourced from the Hartbeespoort dam, could be treated through anaerobic digestion to recover biogas and biofertilizer. However, more experiments are required to fully understand and harness the optimisation opportunities available in applying this technology to manage water hyacinth.

This study evaluated the anaerobic digestibility of primary sludge from two wastewater treatment plants (WWTPs), Leeuwkuil and Rietspruit. Anaerobic biodegradation produces biogas as an energy carrier. Sludge from the primary settling tanks was tested in batch mode as a mono-substrate, without pretreatment or external inoculum. Proximate and ultimate analyses were used to estimate theoretical methane production. Anaerobic digestibility tests were then performed using an Automatic Methane Potential System (AMPTS^® II, Bioprocess Control). The volatile-to-total solid (VS/TS) ratios were 71 for Leeuwkuil and 13 for Rietspruit. Theoretical methane yields for Leeuwkuil sludge were 257–293 L/kg VS. For Rietspruit, the Buswell and Dulong methods gave negative theoretical BMP values (−76 and −15 L/kg VS), suggesting these models may be unsuitable for high-oxygen-content substrates. Measured methane production was 11.3 L/kg VS for Leeuwkuil and 4.8 L/kg VS for Rietspruit, indicating low anaerobic digestibility relative to solid content. Leeuwkuil primary sludge nevertheless showed better potential as a co-substrate for methane production than Rietspruit sludge. Rietspruit sludge may pose challenges for anaerobic digestion, though pretreatment or co-digestion could improve performance. Based on measured methane productivities, each WWTP could generate about 0.5 MWh of electricity per day from biogas. The study shows that primary sludge digestibility depends strongly on the physico-chemical characteristics of the influent wastewater. Primary sludge can often be improved for digestion through chemical/physical pretreatment and co-digestion with secondary sludge or suitable agro-industrial organic residues.

This study investigated the sustainability aspects of implementing a small-scale biogas digester project at the EARTH Centre, a horse-riding facility for the disabled, in South Africa. Firstly, an energy audit of the facility was conducted. From this exercise, energy-saving opportunities through anaerobic digestion of horse manure were identified. Biomethane potential tests (BMPs) were then performed using the Automatic Methane potential test system II (AMPTS II) of BioProcess Control (Lund, Sweden). The horse manure BMP result was 106 L/kg.VS with the biogas averaging a methane content of 40%. This BMP was lower than that of common substrates such as cow manure which can range from 150–210 L/kg.VS. The gas production rate was almost constant in the first 13 days indicating a long hydrolysis period for horse manure. The microbial species in the digester did not change much during the incubation period although small changes were visible in the proportions of each species as the reaction progressed from start to finish. The energy audit showed that 47% of the EARTH Centre’s energy requirements, which equated to 14,372 kWh/year, could be secured from biogas or solar instead of obtaining it from the national grid which is powered mainly by unsustainable coal-fired systems. As a starting point, a 10 cubic meter biogas digester was installed to produce 5512 kWh of energy per year in the form of biogas. To boost biogas production and continue running the system smoothly, it was evident that the horse manure-fed digester would require regular spiking with cow manure as a bioaugmentation strategy. The digester also produced bio-fertiliser and several sustainable development goals were fulfilled by this project. Current efforts are focused on process optimization of this technology at the Earth Centre to further improve the sustainability of the whole business.

Waste management strategies specified in the hierarchy of waste management were evaluated by applying a Multi-Criteria Decision-Making (MCDM) technique called the Analytic Hierarchy Process (AHP) to choose a strategy for managing Johannesburg (JHB) city’s food waste. Under the different weighting scenarios for the evaluation criteria considered in this AHP, the recovery strategy scored an average of 41% while other strategies each scored 33%, 29% and 22% for prevention and reduction, treatment and disposal, re-use and recycle, respectively. Optimisation of resource recovery strategies from food waste is recommended for further consideration and investigation by the JHB municipality in its attempt to promote a circular economy and surmount food waste hurdles.