They comprise of three major components lignin, hemicellulose and cellulose and show the highest availability and lowest costs of all biomasses worldwide. In the vegetation, high fiber contents are found in wood, grass, forestry wastes and agricultural residues The contents of cellulose, hemicellulose, and lignin vary from plant to plant and range from 40 to 60 %, 20 to 40 % and 10 to 25 % of dry weight respectively.
The natural structure of the FOW has high stability and hinders microbial and enzymatic degradation. The resistance of FOW against biological anaerobic and aerobic processes increases with rising lignin content. After anaerobic digestion of MSW or mixed organic wastes, the remaining fraction usually comprises a high share of undegraded lingo-cellulose waste due to its low degradability in absence of oxygen. In order to improve the yields for anaerobic digestion of FOW requires comprehensive pre-treatment methods are required to depolymerize the resistant polysaccharide structures of the fibers and make it accessible for microbial and enzymatic degradation. However, a complete depolymerization of lignin contents is a cost-intensive process.
A residual mixture of bio-solids from the physical, biological and physiochemical treatment of wastewater at STPs, sewage sludge is rich in valuable nutrients, and hazardous to humans and environment due to the presence of heavy metals, chemical contaminants and parasites Therefore, in cities with large sludge generation, controlled sanitization of sewage sludge must be achieved by means of aerobic or anaerobic stabilization. In addition, nutrient recovery from sludge requires further thermochemical treatments.
However, high water content (commonly 95%) makes them unsuitable for traditional processing methods like composting, landfilling or incineration, and increases the transportation costs. Thus, drying of sewage sludge is needed prior to processing. After drying, energy recovery from sludge is also possible, as low moisture content increases their LCV up to 19 MJ/kg incineration enables the breakdown of hazardous chemical compounds and pathogens.
It is the stabilized residue of biogas production through the anaerobic digestion (AD) of organic wastes. After dewatering and drying to reduce volume (lowers transport costs) and to separate valuable nutrients from the high-water content. AD digestate is commonly recycled as fertilizers. Dewatering prior to drying also minimizes volatilization of NH3. The current valorization of digestate as fertilizers results in long storage period, greenhouse emissions, and transportation costs. In addition, digestate from industrial organic wastes causes environmental pollution and health risks to humans due to pollutants in feedstock.
Researches have demonstrated that dried digestate has the potential to be used as a solid fuel with GCV upto 17 MJ/kg or generate syn-gas (with LHV of upto 15 MJ/kg) and biochar (32 wt.%) through pyrolysis Investigation of the characteristics of drying digestate along with sewage sludge and fibrous organic waste are currently lacking. Analysis of co-pyrolysis of sewage sludge, FOW and AD digestate is also not available in the relevant literature.
The issue with urban organic wastes is even more complicated, in megacities like Chennai. Chennai was recently announced as one of India’s first 20 Smart Cities and will have a role model for sustainability, also in terms of waste and wastewater treatment, for entire India. Chennai is one of the most populous cities in India with over 8.5 million inhabitants. The city generates approx. 5,200 tons of MSW per day, of which the Koyambedu Wholesale Market Complex (KWMC), one of Asia’s largest horticulture market, alone contributes approx. 3.5 %.
KWMC in Chennai was inaugurated in 1996 to facilitate easy trade of fruits, flowers and vegetables. One major objective was the decongestion of the city center and to facilitate trading as well as waste collection and disposal. In 2016, the market was spread over an area of 24.3 hectares. It comprises two blocks for vegetable trading and one each for fruit and flower. A current investigation by ISAH and CLRI confirmed that out of the approx. 183 tons of organic waste generated daily at KWMC, 40 % are fibrous banana peduncles. The analysis found that banana peduncles of KWMC comprise with 32.5 % lignin and 24.5 % hemicellulose (based on dry matter) a high share of slowly degrading fibers. Vegetable and fruit wastes contain 17.5 % and 17.2 % lignin respectively.
Fibrous wastes are not only generated in market areas but are ubiquitous in India’s organic household waste through flower garlands, banana leaves and coconuts. In addition to MSW, wastes from KWMC and slaughterhouses are disposed at two dumping sites, Kodungaiyur and Perungudi. Studies state states that both sites are reaching their capacity limits and pose serious health hazards to the citizen.
Within the next 10 years, it is reported that Chennai would generate approx. 1.7 million m³ of sewage per day. Currently, STPs in Chennai have a capacity of 769,000 m³ per day and another two units of capacity 60,000 m3/day are under constructions (Municipal Administration Department, 2016). The entire area of Chennai is connected to the sewer system. Within the large urban areas of India, approx. 30 % is treated before its discharge. Most of the wastewater is directly disposed in the coastal zone of the Bay of Bengal, leading to a severe pollution of the marine flora and fauna. Assuming suspended solids and BOD of 300 mg/L and 150 mg/L in the inlet of sewage, about 250 tons per day of dewatered sludge from STP is generated and disposed in dumping site.
Under these circumstances, there is an urgent need for modern and scientific methods for FOW and sewage sludge treatment for Indian urban areas like Chennai, which also corresponds to the main objectives of the Government of India’s Swachh Bharat Mission
The Smart City Mission is an urban infrastructure development program launched by the Government of India in 2015. The mission aims to “promote cities that provide core infrastructure and give a decent quality of life to its citizens, a clean and sustainable environment and application of ‘smart’ solutions”, in order to “create a replicable model which will act like a light house to other aspiring cities”. One of the key objectives for smart cities is sanitation including solid waste management, sustainable environment as well as health and education. The program fosters sustainable and inclusive infrastructure solutions for tackling contemporary challenges of Indian urban areas.
The pyrasol project intents to develop a sustainable and inclusive solution for FOW and sewage sludge generated in the center of the Smart City Chennai. In doing so, the project tries to identify and utilize synergies for the combined treatment of both waste streams in order to improve the overall treatment efficiency. Both waste streams are redundant non-valuable materials of different physical and chemical characteristics generated within the urban areas. Each waste stream has individual characteristics which negatively effects the treatment.
FOW, however, show a rather low share of nutrients and a high C/N ratio. Biochar from FOW shows a high surfaces area, a high calorific value and an increase nutrient retention potential, but lacks of containing nutrients.
Sewage sludge commonly is high in Nitrogen and other valuable nutrients like phosphorous, whereas the C/N ratio usually is low which influences biodegradation processes. A low surface area and nutrient (and potentially heavy metals) retention potential was observed in the literature for biochar produced out of sewage sludge, due to the sludge low content of structure materials
By co-pyrolysis of both materials, a nutrient-rich, slowly releasing biochar with high surface area is expected to be retrieved. The PYRASOL project addresses synergies of both materials to reduce the individual treatment costs and to create a high value product.
The pyrasol project contributes to the Smart City concept by proposing a zero waste approach. Other presented FOW and sludge treatment approaches (Chapter 2) either create a product which is potentially hazardous (e.g. due to pathogens, parasites) or produce energy and ash from a combustion process, while the ash has to be disposed on a landfill with specific requirements due to its high leaching potential.
By contrast, pyrasol and its combination of sludge drying and pyrolysis ensures an innocuous output product while generating usable energy. The biochar, does not require any further treatment or safe disposal, . It has carbon in stable form which would have otherwise been released into the atmosphere through human activities. Also, biochar can be used as a high value marketable product. By doing so, this project addresses the high sanitary and sustainability standards of Smart Cities in a novel, economic and innovative fashion.