When stabilized through a composting process, sludge can become a good source of organic fertilizer and soil additive, free of chemicals and pathogens. Earthworms feed readily upon the sludge components, rapidly converting them into vermicompost, reduce the pathogens to safe levels and ingest the heavy metals.
Earthworms – the great waste and environmental managers
Earthworms have real potential both to increase the rate of aerobic decomposition and composting of organic matter and also to stabilize the organic residues in the sludge – removing the harmful pathogens (by devouring them and also by discharge of antibacterial coelomic fluid) and heavy metals (by bio-accumulation). They also mineralize the essential nutrients like nitrogen, phosphorus and potassium from the sludge. It may not be possible to remove toxic substances completely, but at least change the ‘chemical make-up’ of the sludge to make it harmless to the soil and enable its use as a nutritive organic fertilizer. This method has been found to comply with grade A standards for sludge stabilization.
Worms are adapted to survive in harsh environments and in moderately acidic-to-alkaline conditions with pH values ranging from 4.5 to 9. They can tolerate a temperature range of 5 to 29°C. A temperature of 20 to 25°C and moisture level of 60 to 75% is optimum for good worm function. They are tolerant to moderate salt salinity and can also tolerate toxic chemicals including heavy metals.
Mechanism of worm action in Vermistabilization
Vermistabilization is a complex mechanical and biochemical transformation of sludge achieved through the action of earthworms. Worms work as a ‘sludge digester’. There are advantages of vermistabilization of sewage sludge by earthworms; the quality of vermicompost upon stabilization is significantly better, rich in key minerals and beneficial soil microbes as compared to the conventional composting which is thermophilic (temperature rising up to 55°C) in which many beneficial microbes are killed and nutrient especially nitrogen is lost (due to gassing off of nitrogen). Worms also keep the system fully aerated and aerobic processes are about 10 times faster than anaerobic processes.
As worms create aerobic conditions in the waste materials by their burrowing actions, the action of anaerobic microbes which release foul-smelling hydrogen sulphide and mercaptans is inhibited. Worms destroy pathogens in the end-product making it pathogen-free. The earthworms release coelomic fluids that have antibacterial properties and destroy all pathogens in the waste biomass. They also devour protozoa, bacteria and fungi as food. They seem to realize instinctively that anaerobic bacterium and fungi are undesirable and so feed upon them preferentially, thus arresting their proliferation.
In the intestine of earthworms, some bacteria and fungi (Penicillium spp. and Aspergillus spp.) have also been found. They produce antibiotics which kill the pathogenic organisms in the sewage sludge making it virtually sterile.
Worms remove heavy metals and toxic chemicals from the end-product. Earthworms (especially E. foetida) can bio-accumulate high concentrations of metals including heavy metals in their tissues without affecting their physiology and this particularly when the metals are mostly non-bio available. They can readily bio-accumulate cadmium, mercury, lead, copper, manganese, calcium, iron and zinc and extremely high amounts of Zn, Pb and Cd. Cadmium levels up to 100 mg kg-1 and lead up to 7600 mg g-1 dry weight have been found in tissues.
Worms decrease total organic carbon (TOC) and lower C/N ratio of sludge. This has significance when the composted sludge is added to soil as fertilizer. Plants cannot absorb and assimilate mineral nitrogen unless the carbon to nitrogen (C/N) ratio is about 20:1 or lower. Mineralization of organic matter in sewage sludge by earthworms leads to a significant decrease in total organic carbon (TOC) content thus lowering the C/N ratio. Worms reduce volatile solids from sludge.
Low greenhouse gas (methane) emission by vermicomposting
Biodegradation of organic waste has long been known to generate methane. Vermicomposting by worms decreases the proportion of anaerobic to aerobic decomposition, resulting in a significant decrease in CH4 and volatile sulphur compounds which are readily emitted from conventional microbial composting.
Worm biomass: a nutritive meal for the fishery, poultry and dairy industries
Large-scale vermicomposting of sludge would result in the production of tonnes of worm biomass every year which is a ‘pro-biotic’ food for the poultry, dairy and fishery industries. ‘Worm meal’ is rich in the essential amino acids lysine and methionine.
Problems encountered during vermicomposting of sewage sludge
There is initial odour problem because the worms are overloaded with waste beyond their ‘carrying capacity’ at a given time and oxygen supply becomes insufficient leading to anaerobic conditions. As the worms grow and multiply over a period of time, they discharge more anti-pathogenic coelomic fluid, create more aerobic conditions in the waste biomass by burrowing actions, and devour the anaerobic microbes thus removing the odour problem.
However, a natural insoluble mineral zeolite (aluminium silicate) when mixed with sludge (3 to 5%) reduces or eliminates the foul odour by absorbing the gases ammonia and hydrogen sulphide. Zeolite has additional advantages. Its cage-like skeletal structure traps heavy metals and prevents their leaching into the environment. It also helps to raise or lower pH of waste through cation exchange.
Environmental and economic benefits of vermistabilization of sewage sludge
Vermistabilization by vermicomposting involves about 100 to 1000 times higher value addition in the system. Earthworms can compost and convert municipal and industrial sewage sludge into a valuable resource at a fraction of the cost of other methods and also divert them from ending up in landfills.
Landfills are proving to be an economic and environmental burden (emitting greenhouse and toxic gases and the threat of groundwater contamination by leachate discharge) for nations. Classed as a biohazard, sewage sludge has to be necessarily disposed in secured landfill sites at still higher cost. The up-front cost of construction of an average secured landfill is estimated to be US$ 15 to 25 million before the first load of waste is dumped. This explains the dollar value of diverting every cubic metre of sludge from the landfills by vermicomposting.
Some nations incinerate sewage sludge and the cost of incineration is both economic (in terms of high use of energy) and environmental (in terms of pollution). Australia is saving over 13,000m3 of landfill space every year by vermicomposting.
The Hobart City Council in Tasmania is saving AU$56,000 per year just from avoiding landfill disposal and earning an equal amount (about AU$55,000) from the sale of vermicompost to the public.
Economic and environmental benefits also accrue from the production and sale of live worms and their excreta (vermicompost). Together, they significantly improve soil fertility and food productivity is enhanced by over 30 to 40% as compared to chemical fertilizers. The pharmaceutical industry is also finding a new use for earthworms in the manufacture of antibiotics from their anti-pathogenic coelomic fluid.
- Vermistabilization process driven by the earthworms tends to become more robust and efficient with time as the number of degrader worms grows and invades the waste biomass.
- Sewage sludge is a potential health hazard as it contains high numbers of cysts of protozoa, parasitic ova, faecal pathogens like Salmonella spp., Shigella spp. and Escherichia coli and also heavy metals such as zinc, cadmium, mercury and copper. In addition, sludge contains organic molecules and essential plant nutrients like nitrogen, phosphorus, potassium and various trace elements.
Holistic approach towards conversion of sewage
Vermicomposting is a self-promoted, self-regulated, self improving, self-driven, self-powered and self-enhanced, low or no energy requiring zero-waste technology, easy to construct, operate and maintain. The worm number and quantity (biomass) is a critical factor for vermicomposting/vermistabilization of all solid wastes besides the optimal temperature and moisture which determine worm activity. A minimum of about 100 to 150 adult worms per kilogramme of waste would be ideal to start with for rapid biodegradation and also an odour-free process.
Vermiculture technology can even prevent formation of sewage sludge if the waste-eater earthworms are used in sewage treatment plants. They feed upon the waste-water solids besides significantly reducing the BOD and COD loads. Practicing vermicomposting technology for management of all organic wastes (solid and liquid) including sewage sludge is like ‘killing two birds in one shot’. It can provide solutions to the twin objectives of safe waste management and sustainable agriculture by converting all the societal waste organics into nutritive biofertilizer which will be used by farmers to grow more food for society.The article has been extracted from the research study authored by Rajiv K. Sinha, Sunil Herat, Gokul Bharambe, Ashish Brahambhatt from Griffith School of Engineering (Environment), Griffith University, Australia