Solid Waste Management Biotechnology

Global municipal solid waste generation exceeds 2 billion tonnes per year and is growing rapidly, particularly in low- and middle-income countries undergoing urbanization and economic development. At least a third of this waste is organic — food waste, yard waste, paper, cardboard — and is amenable to biological treatment. Yet most organic waste is still landfilled or openly dumped, generating methane emissions, leachate, and public health risks. Biological treatment offers sustainable alternatives that recover value from waste.

The major biological treatment technologies are composting (aerobic decomposition to a stabilized soil amendment) and anaerobic digestion (anaerobic decomposition to biogas and digestate). Each can be deployed at scales from household to municipal, with different feedstocks, process configurations, and end products. Both technologies have long histories but continue to evolve through research and innovation.

Hrisana Journal welcomes submissions across all aspects of biological solid waste management: process microbiology, reactor engineering, feedstock pre-treatment, product quality and safety, emissions control, life-cycle assessment, and integration with waste management systems. Our interdisciplinary scope reflects the systemic nature of waste management challenges.

Composting is the aerobic biological decomposition of organic matter to a stabilized, humus-like product. The process passes through mesophilic, thermophilic, and curing phases, with temperatures reaching 55-65°C during the thermophilic phase. These high temperatures kill pathogens and weed seeds, making the resulting compost safe for agricultural use. The mature compost is a valuable soil amendment, improving soil structure, water retention, and nutrient content.

Key process parameters include the carbon-to-nitrogen (C:N) ratio (ideally 25-30:1), moisture content (50-60%), aeration (oxygen concentrations above 10%), and pH (typically self-regulating). Feedstock composition determines C:N and other properties: food waste is nitrogen-rich (low C:N), while yard waste and paper are carbon-rich (high C:N). Blending feedstocks to achieve the target C:N is essential for process performance. Aeration can be provided by turning (windrow composting), forced air (aerated static pile), or in-vessel systems.

Process challenges include odor emissions (particularly ammonia and hydrogen sulfide), volatile organic compound emissions, and greenhouse gas emissions (methane from anaerobic pockets, nitrous oxide from incomplete denitrification). Odor control is often the most important consideration for composting facilities located near populated areas, with biofilters, enclosed reactors, and careful process management all playing a role. Greenhouse gas emissions can be minimized through proper aeration management, moisture control, and avoidance of anaerobic pockets.

Anaerobic digestion (AD) of solid waste converts organic matter to biogas and digestate under anaerobic conditions. Compared to composting, AD offers the advantage of energy recovery (biogas), reduced odor emissions (closed reactors), and smaller footprint. The technology is well-established for sewage sludge and agricultural waste and is increasingly applied to food waste and the organic fraction of municipal solid waste.

Feedstock preparation is critical for solid waste AD. Source-separated food waste can be fed directly after minimal pre-treatment (particle size reduction, removal of contaminants). Mixed municipal waste requires mechanical-biological pre-treatment to separate the organic fraction from recyclables and rejects. The organic fraction is then slurried with water or digestate to a pumpable consistency for wet AD, or processed at higher dry matter content in dry AD systems.

Co-digestion — combining multiple feedstocks — often improves performance by balancing nutrients, diluting inhibitors, and optimizing the C:N ratio. Food waste co-digested with sewage sludge or animal manure achieves higher biogas yields than either feedstock alone. Process stability depends on maintaining the syntrophic balance between acid-forming and methane-forming microorganisms; monitoring of volatile fatty acids, alkalinity, and methane content provides early warning of instability. Post-digestion, the digestate is dewatered with the liquid fraction recirculated or used as fertilizer and the solid fraction composted or used directly as a soil amendment.

Bio-drying uses the heat generated by aerobic microbial activity to dry waste, reducing its mass and volume and producing a refuse-derived fuel. The process operates with high aeration rates to remove moisture as water vapour, with retention times of 7-15 days. The dried material has a higher calorific value than the raw waste and can be combusted for energy recovery or landfilled with reduced leachate generation. Bio-drying is particularly suited to high-moisture wastes such as food waste and sewage sludge.

Mechanical-biological treatment (MBT) combines mechanical sorting with biological treatment to process mixed municipal solid waste. The mechanical stage recovers recyclables (metals, plastics, glass) and produces a high-calorific refuse-derived fuel stream. The biological stage stabilizes the organic fraction through composting or anaerobic digestion, producing a stabilized material suitable for landfill cover or, in some jurisdictions, land application. MBT plants are widely deployed in Europe as an alternative to direct landfilling.

MBT reduces the environmental impacts of waste management by recovering materials and energy, stabilizing organic matter before landfilling, and reducing landfill methane emissions. However, the complexity and capital cost of MBT plants, the quality of recovered materials (often contaminated), and the markets for end products (refuse-derived fuel, stabilized organic matter) all influence economic viability. Life-cycle assessment is essential for evaluating MBT against alternative waste management approaches for specific local contexts.

The waste biorefinery concept extends biological waste treatment beyond compost and biogas to produce higher-value products. Food waste can be fermented to lactic acid, succinic acid, or other platform chemicals; volatile fatty acids from acidogenic fermentation can be converted to bioplastics (PHA) by specialized bacteria; microbial lipids from oleaginous yeasts can be converted to biodiesel. These approaches can improve the economics of waste treatment while displacing fossil-based products.

Integration with sanitation and agriculture closes resource loops. Source-separated human excreta can be treated to recover nutrients and energy, reducing the environmental footprint of sanitation while supporting agricultural productivity. Municipal wastewater treatment plants can be transformed into resource recovery facilities producing water, energy, nutrients, and materials. These integrated approaches require coordination across sectors and infrastructure systems but offer the potential for more sustainable urban metabolism.

For researchers, biological solid waste management offers rich opportunities spanning microbiology, process engineering, environmental science, and sustainability assessment. Hrisana Journal welcomes submissions across this spectrum, particularly work that integrates technical, environmental, and economic perspectives. Visit our Submit Manuscript page to begin your submission, or review our Author Guidelines for preparation requirements.