Black and Veatch's Frank Rogalla looks at biosolids vermistabilisationAt present, approximately half of the UK's sludge production is recycled to agriculture. If is has come from large STWs is has usually been anaerobically digested and de-watered, sludge from small rural STWs, however, has traditionally been raw and untreated.
Therefore low-cost sludge treatment processes, with minimal operator input, are needed for small STWs. Sludge vermistabilisation, using worms to process sludges potentially meets this need. It converts the solids into valuable fertiliser or soil conditioner, with the following advantages:
- fragmentation of sludges, increasing surface area available for drying and microbial decomposition,
- decrease in particle size and thus increase in moisture holding capacity,
- improved aeration due to tunnelling action of worms,
- increased concentration of available micro and macro nutrients,
- reduction in carbon: nitrogen ratio,
- production of a soil conditioner with a rich, humus odour.
Maintaining the correct physicochemical environment for worm populations is vital. In general, the compost media must be aerobic and the ionic conductivity should be less than 3mS/cm. Physical parameters including pH, electrical conductivity and temperature must be controlled and monitored for optimum worm health and consistent product. For optimum productivity, a worm population requires a fresh and regular supply of feed substrate into which it can move. This lends itself to continuous flow type systems.
Other process parameters include:
- optimum solids concentration of the feed sludge, and a 5.5 - 8.5 pH range,
- frequency of feeding and sludge loading rates,
- optimum bed depths and worm population densities,
- temperature and other climatic controls,
- light management,
- health and safety, operator access and ease of maintenance.
In 1984, US National Science Foundation funded, laboratory-scale vermistabilisation research at Cornell University, found capital and operating costs to be competitive with other biosolids management systems. In the UK, a prototype, fully-automated vermistabilisation unit was commissioned in April 1999 at Clayton West STW in south Yorkshire, financed by five utilities. A final design could be as simple as a rectangular drying bed with standard drainage, using an easy-to-remove lid and low-specification motor. The feeding regime with a relatively constant solids value between 1.5-2.0% dry solids was stabilised at a daily average solids loading of 0.5kg/m²/wk.
Effluent analysis is important for both costing the liquor treatment and as a process performance indicator. The effluent quality improved as the process stabilised, with typical solids concentration around 50mg/l and ammonia between 2-12mg/l. Neither snow and ice nor in-tank temperatures exceeding 30°C appeared to cause any significant operational problems. However, extended operation at low temperatures over a winter season is expected to decrease overall worm activity.
Making the grade
In Australia three facilities have been operating for about four years, two processing piggery waste (100 and 150 wet tonnes/w respectively) and a third processing 250 wet tonnes/w of biosolids. The vermiculture facility at Redland Bay is licensed by the Queensland Environmental Protection Agency (EPA) and is fully hazard analysis and critical control point (HACCP) certified. The final product must achieve grade A stabilisation as defined in the New South Wales EPA Use and Disposal of Biosolids Products 1997 regulations. Sludges from several STWs with a range of treatment and de-watering techniques are mixed on receipt and fed directly to the surface of the beds with no pre-treatment.
Traditional wisdom has it that pathogens in sewage sludge: E.coli, Salmonella, faecal coliforms, viruses and Helminth ova can only be rapidly destroyed by high temperature and/or high pH. Yet, during three months, for faecal coliform input levels of 300,000MPN/g, the output colony count was less than 200MPN/g, compared to the New South Wales EPA grade A requirements of less than 1,000MPN/g. E.coli counts were reduced from an 167,000MPN/g to 100MPN/g, well below the maintenance requirements desired for an enhanced treated sludge under the Safe Sludge Matrix. No Salmonella was detected in the output. Enteric viruses and Helminth ova were recorded as <1/g.
One of the important aspects of sludge management is the reduction in volatile solids (VS). Replicated trials at Redland have established a VS reduction, on undigested biological nutrient removal (BNR) sludge in excess of 50%.
At a loading rate of around 4kg ts/m²/d, transit time through the system is 40 days. By way of comparison the USEPA indicates die-off rates for pathogens in soil is at minimum two months for bacteria and three months for viruses.
What makes vermiculture potentially significant is not just its ability to reduce pathogens and volatile solids. Most important is the potential value of the end product as every water utility in the world struggles to find sustainable solutions for sewage sludge management. EU policy encourages beneficial re-use with a focus on land application while some jurisdictions debate over the safety of sludge use in agriculture.
The agricultural value of the vermicast end product has been explored in numerous
trials. The research results achieved in Australia and USA ranged from increased
crop yield to suppression of plant diseases, obtained at very low application
rates (<2t/Ha). In addition to lowering nutrient contamination and soil metals
loading, vermicast looks and smells like fine soil.
Perhaps one of the great hidden benefits of vermiculture is its environmentally friendly pedigree. As we struggle with approvals, licensing and public opinion, we may now worm our way through the approval process, reducing fears on sludge use and providing a cost-effective long-term solution.