Black and Veatch's Frank Rogalla looks at biosolids vermistabilisation
At 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
- 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.