VERY LARGE SCALE VERMICULTURE IN SLUDGE STABILISATION
Author: M Lotzof
Affiliation: Vermitech Pty Limited
The use of vermiculture as a method for the stabilisation of sewage sludge and other organic wastes has been mooted for the past 50 years. Considerable work on the stabilisation process and the value of the vermicast by-product has been carried out)
Notwithstanding the scientific support for the process, vermiculture has remained a cottage industry, with very few large scale facilities processing substantial quantifies of organic waste. The barrier to the commercial implementation has been a mixture of engineering, capital and perceived risks of an unproven process and the high cost of operating traditional on ground systems. Much of the fault lies with the vermiculture industry which has focused on the breeding of worms for sale, rather than on the business of waste management and vermicast marketing.
The driving force behind the introduction of vermiculture, or other reuse processes, is the global recognition of the need to recover organic material and return this to the soil. Legislation is being enacted to prevent the dumping of organic material to land fill. Simultaneously, the cost structures of dumping are increasing and farmers are becoming more aware of the need to change their practices to halt and reverse the degradation of their soils. There is thus market pressure for the waste processing and the consumption of the end product.
To be a viable alternative, very large scale vermiculture must be proven to be ecologically and commercially sustainable, capable of being operated without subsidy, on a competitive basis.
Vermitech has, through the development of proprietary equipment and processes, created a system that can consistently and cost effectively, stabilise a large range of organic wastes including sewage sludge. This paper will describe the process and systems, the regimes for verifying the achievement of required stabilisation and contamination, the design criteria for ensuring compliance with environmental impact restrictions and finally the value of vermicast as an end product.
The critical elements of very large scale vermiculture are:
ú the preparation of the sludge prior to feeding to the worms ú the controlled application of the feed to the worm beds ú the raised cage bed structures
ú the environmental control systems for managing moisture, temperature and wind ú worm biomass management ú harvesting of the vermicast
ú post processing of the vermicast for sale
ú leachate control
ú sampling and analysis systems for sludge and vermicast
ú information management systems
The system is operated within a quality controlled environment where the
parameters are set by the regulatory authorities, the requirements of the local
authorities and the end use customer.
Sludge from a broad spectrum of sewage and water treatment plants is being stabilised and the end product sold. Dewatered sludge is taken directly from the treatment plant and fed to the worms without the need for any pre composting or aging.
Systems have been installed on a number of sites, the largest being a 400m3/week capacity facility at Redland in Brisbane Queensland. The system at Redland was selected after a competitive tender. The contract was awarded on the basis of lower cost, superior process and greater end product potential.
Testing has established that the vermicast meets the Grade A Stabilisation Standards. Contamination levels while dependent on input contaminants, are managed so that only Grade A or B level vermicast will be marketed. Contamination levels are controlled by analysing incoming sludges and blending in "clean" organic material to the feed mix to reduce the ultimate level.
Extensive trials have established a very large market for vermicast. Grower results across a large range of crops, indicates a very large, sustainable market with pricing exceeding $250/m3.
Very Large Scale Vermiculture offers an ecologically and commercially sustainable alternative to cocomposting or lime stabilisation. The major advantages are:
Vermiculture, worm farm, vermicast, stabilisation, sewage sludge, waste reduction
Vermiculture is the process by which organic material is fed to a variety of worm species with the purpose of converting the organic material into increased worm biomass and vermicast. Vermicast is the excreta from worms and has use as a plant growth medium and soil conditioner. The worm biomass has been sold for bait, animal feed and domestic and small composting systems.
Worms have an ability to convert a wide range of organic material including sewage sludge provided that the material is presented in an acceptable form. Many laboratory scale experiments were carried out and many small scale operations exist. However the work on sludge was seen as too difficult because of the need to perfect stabilisation techniques and manage contamination. Both require extensive and expensive research and sampling and analysis.
The driving force behind the search for more effective recycling systems for sludge is a global perception of the need to recycle. Zero organic waste to landfill targets coupled with prohibitive charges on dumping organic waste are the tools to turn the perception into a reality.
2. THE REDLAND SYSTEM - QUEENSLAND AUSTRALIA.
Redland Shire Council, issued a tender in 1997 for the disposal of 250m3/week of sludge from 4 sewage plants and one water treatment plant. The tender required that the selected system remove and process sludge on a continuous basis. Three types of processes were considered, composting, lime stabilisation and vermiculture.
3. THE SITE
The site has been licensed by the Queensland Department of Environment. An Integrated Environmental Management System was prepared for the site as part of the licensing requirements.
The installed system consists of a central worm farm on the Cleveland STP. The collection from each of the five treatment plants is by well proven covered hook lift mounted sludge bins.
The worm farm is divided into two areas, the worm bed/waste receival area and the vermicast storage/ post processing area. The worm beds occupy an area of 100m x 80m. The surface is bitumen sealed and drains to a leachate dam with first flush control.
4. THE BEDS
The beds are galvanised steel frame with the waste and worm biomass contained within a raised mesh cage. The 14 beds are each 3.6m wide and 70 m long. The beds are modular and can be configured to any length. At Redland, the total available surface area for feeding exceeds 3,000m2 giving a capacity of 400m3/week. The additional 150m3/week capacity was put in place to enable the worm farm to process stockpiled sludge and to take other regional wastes.
The raised cage system illustrated in Figure 1, is a continuous flow process.
Waste is fed to the surface. The worms progressively stabilise the material. The
fully stabilised material is harvested from the base. The design maximises the
retention of the worm biomass, eliminating the need to separate the worms from
the vermicast. It also optimises the environment to promote the development of
beneficial bacteria and fungi.
Raised Cage Design
The waste from the five sites is received into a bunded mixing area. Prior to the commencement of operations, sludge was collected from each of the five plants and fed to the worms over a six week period to determine the correct blending to ensure maximum attractiveness of the sludge to the worms.
Each waste has its own blend requirement. The objective of blending is to deodorise and aerate the waste and adjust the Carbon/Nitrogen balance, the pH and salinity. A range of mineral, organic and bacterial additives may be mixed depending on the nature of the waste material and the state of the worm beds. The formulae are proprietary. The mixing has made all sludges worm accessible, even some "specially aged" material prepared for us for an odour/eatability trial. The standard practice of collection, blending and feeding on the same day minimises the potential for any odour build up.
Worms seem to 'be quite resilient to heavy metals contamination, Dieldrin and PCB's 1.1,1.2. To date we have had no contaminant based poisoning, though we have destroyed worm stock by unintentionally adding caustic sludge from a food processing plant. It is clear that worms do bio-accumulate heavy metals, but the degree and their value as toxic purifiers is being researched.
The worm beds once fully populated, are fed across their entire surface on a daily regime. Controlling the depth of feeding is critical to the process. If too much is fed, or if it is fed to thickly, there is potential for the material to compost, or turn anaerobic. Both conditions prevent worm activity. It is thus essential that the quantity fed match the daily quantity consumed by the worms.
Worms eat between one half and their own body weight per day. On this basis, the Redland site will contain between 80 and 160 tonne of worm biomass.
One of the major limitations with on ground worm farms is the high potential for the formation of aerobic zones which contaminate the material promoting pathogen re-growth. This is avoided in the raised bed design which maximises the airflow into the bed and facilitates an even distribution of mixed sludge.
7. THE STABILISATION PROCESS
The grinding and tumbling action within the worm gut reduces the sludge particle size exposing a greater surface area to a range of viruscidal enzymes and a host of bacteria. After excretion the sludge particles continue to be exposed to an aerobic environment in the raised cage beds with very high bacterial and fungal populations. Stabilisation is progressive down through the bed. The chart below (Figure 2) illustrates the typical pathogen reduction profile from raw waste through to stockpile vermicast. The exact stabilisation mechanism has not be quantified, but the empirical results are clear.
All vermicast is rigorously tested to ensure compliance with the regulatory guidelines (Table 1). All results from the raised beds have met Stabilisation Grade A.
Vermiculture can be used to stabilise any sludge. However it is more appropriate for processing ,sludges which meet A or B grade contamination. More highly contaminated wastes, while capable of being stabilised may result in a level of contamination of the vermicast which renders the product unsafe for uncontrolled use.
Vermicast is applied at relatively low rates. Less than a tonne to the hectare is the recommend rate for all but the most degraded soils. The contamination and subsequent application control standards vary across jurisdictions.
At Redland most contaminants fall within Grade A reflecting the relatively low industrial base. Only Copper and Zinc push the vermicast to Grade B. Given the low application rates and the common agricultural use of both these elements, it may be that a new standard for vermicast grading needs to be developed.
While worms survive and even flourish in contaminated wastes, to date there is insufficient information to determine what bio-remediation qualities they possess. Even if there is some uptake of heavy metals, the degree is not quantified. Similarly there appears to be a decrease in the levels of PCB's and organo-phosphates, but again the data is anecdotal and the method of destruction is not known. We are conducting research in this area and results will be published in 6-12 months.
9. SAMPLING & TESTING
The sampling and testing regime was established to ensure compliance with the regulatory and contractual requirements. Both the raw sludge and the vermicast are tested by extemal NATA laboratories. In addition we conduct a range of analysis on site to ensure that the waste mix complies with the blend formulation requirements.
Samples are taken of each bin of sludge. Weekly composites are sent for analysis. In order to track the source of abnormal results, individual bin sub-samples are kept refrigerated until the composite result is obtained. Council will be provided with all the data from all the sampling.
Data from sampling is used to predict contamination levels and to assist the Council in tracking the source of contaminants. For example we identified a dramatic increase in the amount of Selenium. The rise plateaued and stabilised. A new battery reprocessing facility was established in the time frame of the increase. Their practices are being investigated.
In another incident a Chromium user advised of an accidental discharge into the sewer. Sludge from the plant was quarantined and tested. No increases above the Grade B input levels were detected and the sludge was processed normally.
The vermicast end product is sampled and tested following the regulatory and contractual requirements.
10. THE VERMICAST END PRODUCT
Vermicast is worm excreta. But it is completely different to the original sludge. Vermicast harvested from the base of the bed will have been in the bed in excess of 80 days. It is odourless, smelling like good soil. It will be fully cast, free from live worm and viable eggs. Post processing consists of windrow drying under cover, blending for quality and screening to obtain a uniform product. The product is not sterilised or pasteurised, but meets all stabilisation criteria. Final pH is in the range 6.37.2. CEC exceeds 30.
As part of the contract, a detailed Stabilised Sludge Management Plan governing testing, quarantining, blending, labeling and dispatch procedures was prepared.
Vermicast is not sold by specification as a fertiliser. It is sold as a soil conditioner by generic type. Vermicast quality will vary according to the food source, the production process used and the post processing practices. Vermitech is establishing internal standards for the BioVermTM product to differentiate it from generic vermicasts and to develop product reliability standards for the consumer. Most trials on vermicast growth properties have not identified the chemical or biological composition, the food source or the method of processing.
Considerable anecdotal information exists. There is also extensive use of vermicast in Cuba and India and these results have been obtained. The lack of comprehensive scientific research is being addressed. Dr J Buckerfield (3) of the CSIRO in Adelaide has conducted a broad range of tests as has the Australian National University (4). Some of the results are illustrated below (Figure 3).
Vermitech has commissioned research by The University of Sydney, CSIRO, Departments of Agriculture and Primary Industry and large commercial growers. All the trials are standardised replicates designed to establish firstly the commercial value of BioVermTM to growers of different crops in different regions with varying soils. Over 50 trials are under way at the time of writing.
Interim results using BioVerm have demonstrated:
The research papers will be published once the trials are complete.
11. THE BIOVERM MARKET
BioVerm is being targeted at high value horticulture, viticulture and seedling propagation. Oversee over 150,000 tonnes is used annually in areas such as sugar cane, tobacco, coffee and table grapes BioVerm is being marketed as a soil conditioner, not a fertiliser, though in some countries with k regulatory control, vermicast is seen as a fertiliser. BioVerm is recommended to be used in conjunction with standard farm practice, or as an additive to seedling and potting mixes for the nursery industry. is accepted that only a small percentage of the total fertiliser applied to the soil is taken up by the plants. The rest is washed into the rivers, polluting surface and underground water systems, or locked up in the soils. Once the impact on nutrient uptake is quantified, "standard" farm practices c be amended to reduce the amount of fertilisers applied.
There is a growing recognition by broad-acre farmers that their soil is being severely degraded. They have been compensating by steadily increasing the quantity of inorganic fertilisers. BioVerm is being positioned as one part of the rehabilitation process returning soil bacteria and fungi on an organic substrate.
The annual fertiliser market in Australia exceeds 2.5 million tonnes. Vermitech's target is less than 2%. The impact of the Redland production is negligible.
Unlike composts, dried manures and mulches, BioVerm commands a price exceeding $250/m3 FOB the worm farm for bulk sales. The non-sewage based vermicast commands a retail premium selling 1 $1 per kilo RRP. The non-sewage product has also be certified as Organic by Biological Farmers Association and a premium may be secured from Organic Farmers.
12. COMPARISON VERMICULTURE, LIME STABILISATION AND CO-COMPOSTING.
It is assumed that the operating costs are similar given the handling and
mixing requirements of three systems. Given equal operating costs, very large
scale vermiculture has some distinct advantage
Very large scale vermiculture is a capital intensive activity. Development of
the technology and the vermicast market has absorbed millions of dollars in
research and development and will continue to do
On a stand alone basis, a facility the size of Redland provides an investment
that yields a superior return to most infrastructure projects. The rate of
return is governed by:
From this approximately 7,000 m3 of vermicast will be produced at a wholesale value of $1.7 million. Commercial prudence requires that the difference between the operating costs and fees charged to council remain confidential.
This situation provides an opportunity for sludge producers to enter into arrangements whereby, through the co-operative development of very large scale vermiculture operations, the cost of waste disposal can be turned into a profit centre.
Very large scale vermiculture parallels the concentration of waste production as a result of greater urbanisation and the development of intensive agriculture. The very large system concentrates natures biological cleaning agents - worms, bacteria and fungi into a continuous flow process. The scale of the facility is simply a function of the quantity of the waste to be processed.
The process meets, or exceeds all regulatory requirements providing a publicly popular solution to the problem of sludge disposal. The process is cost effective for waste producers allowing them to meet even the most stringent waste reduction targets without any increase in costs.
The vermicast end product is superior to other re-use products providing greater ease of use, benefit and hence value to the agricultural consumer.
As a co-operative venture, very large scale vermiculture can turn the cost of waste disposal into a profit centre for waste authorities.
1. "Earthworms in Waste & Environmental Management Edited" by C Edwards
& E Neuhauser 1998, SPB Academic Publishing
1.1. The Potential of Earthworms for Managing Human Sludge Neuhauser et al
1.2. The Prediction of Field Toxicity of Chemicals to Earthworms G C Goats & Edwards
1.3. Earthworm toxicological tests.. M B Bouche
2. Gutteridge Haskin & Davey, Press Release and quoted in "Crosscurrent" December 1997
3. John Buckerfield, Researcher, CSIRO, Adelaide