Covered Aerated Static Pile Composting
Background
Composting in the UK has previously been mainly by open windrow. Whilst this system relies upon mechanical turning of the material to maintain porosity and aeration, the alternative Aerated Static Pile (ASP) system uses a pump to force air through the pile to aid the composting process. ASP composting systems require less land area than needed to compost equivalent quantities of material by open windrow, but require capital investment in pipework and air supply.
 DEFRA and the Environment Agency have indicated that the issues of bioaerosols, odour control and the recent foot & mouth outbreaks may drive composting of particular wastes towards containment in the UK, especially where difficult or odorous wastes are received, or there are neighbouring sensitive receptors. This is generally seen as a trend from open windrow and ASP towards in-vessel systems, which although they offer containment and process control, are much more expensive to establish and operate on similar scales.
Covered ASP can be an economic and technically acceptable alternative to in-vessel systems, while still achieving the same aims. The Gore™ membrane cover system significantly reduces migration of odours and bioaerosols, reduces leachate and prevents rainwater ingress and vermin access. The cost of the covers is approximately equivalent to the cost of the aeration infrastructure.
Benefits of covered ASP
The Gore™ system of covered ASP appears to have different advantages over both turned windrow and in-vessel systems. Advantages over turned windrow are that the system provides containment of both odour and bioaerosols, and prevents access to scavengers, all currently of concern to regulators. At the same time, it offers a degree of process control, through the automatic introduction of air in response to oxygen monitoring in the pile.
In comparison to in-vessel systems, we expect that the Gore™ ASP system costs less than containerised, bay, box , tower or cage in vessel systems, and is a simpler technology, potentially with less to go wrong. It appears relatively easy to change the scale of operation by introducing more covers and pipework, and the Gore system can be operated on a larger scale (more akin to windrow composting sites) without the need to erect buildings.
Case Study
This case study considers four sites around Baden Baden in Germany which employ the Gore™ cover system; two are operated by Vogel Compost, which operates three dedicated composting facilities, employing 15 staff and accepting more than 70,000 tonnes of organic wastes annually, through a series of long term, high value municipal contracts. The principal feedstocks are green waste or a combination of green waste and pre-treated kitchen waste, and operations are tailored towards the production of a high quality product range. It is important to note that much of the success of the facilities visited could be attributed to the skill and experience of the site managers, as well as the employment of the cover and aeration systems.
Vogel market a wide range of products (in both bulk and bag) from their sites; ~10% of product goes into private gardens, 90% into agriculture (primarily strawberry, asparagus, wine – all high value crops). 10-30mm or 20-30mm compost is marketed to vineyards for moisture retention and erosion control. Also, tree bark is shredded at the site and screened to <20mm for sale as mulch.
The other two sites considered are co-located with agricultural and wastewater treatment businesses respectively.
Site 1: Covered ASP at Buhl Site (Vogel)
"A technology which speeds up biological processes, yet is much less expensive than previous approaches"
Photo 1 - Buhl Site
Image shows a covered ASP trial in Germany
The Umwelttechnik Vogel (UTV) brochure states:
"Covered aerated static pile systems can provide the containment which regulators are striving for, at the scale of open windrow and without the expense of in-vessel systems"
Photo 2 – Wewa site
Image shows a covered ASP trial in the UK
The Buhl facility is Vogel's "show site" accepting 20 ktpa of green waste delivered by the local authority, householders and landscapers; this is very clean and well segregated. Infrastructure at the 1 hectare site includes concreted surface, weigh-bridge, covered storage & site offices. The site shares a shredder/chipper with Vogel's other 2 sites, has a trommel screen, bucket loader, logbreaker and bagging plant and employs 4 staff. The stages of the process are pre-rot, then intensive rot, curing, followed by screening, blending and bagging. Clean green waste is segregated into grass, leaves, wood (twigs etc) – mixed, shredded and left to stand (pre-rot) for 1-2 weeks. The intensive rot (aerated static pile under cover) period is a total of 8 weeks (4 weeks, turn, 4 weeks), curing is ~4 weeks (no turning).
Screening, blending and bagging arrangements differ by product. Screen overs are returned to the composting operation. Covered ASP is used in the intensive rot area, which has air compressors and in-floor ducts (2/pile); piles are 8m wide x 3-3.5m high x 30m long (200t or 300 m3 per pile) [nb can do 50m long pile with 1 pump & 2 pipes]. The cover system is rolled out using a wall-mounted roller, which can be operated by one person (remote) or 2 (by hand). The piles have automated aeration (1.4 kWh fans), based on PC-controlled O2 monitoring [1xO2 sensor (at 1m depth) and 1 temperature probe (measuring at 5 points) per pile].
Site 2: Baden Baden
The Vogel site at Baden Baden is similar to Buhl, but accepts a range of wastes (15 ktpa green waste, 7 ktpa biowaste from collection of kitchen (not green) waste, post separation). Also accepted is clay powder from hospitals, paper beer labels and hemp. The site has hard surfacing, a shared shredder / chipper, screens, covered storage, office, bagging plant, bucket loader, remote control combined high velocity shredder / turner (increases defibration)), tractor & spreader. The intensive rot area has compressor & on-floor pipes (150mm dia HDPE @ 6-7m c/c), and Gore™ covers with mobile cover roller. The process is different due to the nature of the wastes - green waste is shredded and left for up to 2 weeks (no turning); mixed 70% green waste to 30% biowaste; intensive rot period is 4-5 weeks – ASP under cover, turned once; turn into curing piles; curing (with aeration) period 8-10 weeks; screening & stockpiling (aerated). Products sold into high quality markets.
Sites 3 & 4. Wewa and Stupferich sites
A farmer operates the Wewa site as a secondary business, accepting sewage sludge cake from a small village AD plant (5,000 population), green wastes plus residuals (sawdust and oversize woodchip) from a neighbouring timber plant. Assumed internal or low value agricultural market for product. Covered ASP using Gore™ system, on-floor aeration (pipes covered by woodchip), mixing with bucket loader – 4 weeks approx. intensive rot phase. The aims of this site are odour reduction (neighbour issues) and pasteurisation, compared to waste management sites, where the aims are volume reduction and stabilisation.
The Stupferich site is co-located with a wastewater treatment plant serving the City of Karlsrühe; it receives 8,000 tpa (50 tonnes per week) of separated (post BTA) biowaste and woodchip (as bulking agent / carbon source), plus oversize material from a windrow. A walled Gore system in use at this site – dimensions are 12m by 8 m x 2.4 m high wall. Concrete walls and floor, combined in-floor aeration and drainage system, Gore membrane in the doors and opening roof). 2 weeks active rot in the system, then windrow outdoors to cure.
About ReMaDe
ReMaDe (REecyclates MArket DEvelopment) initiatives are regional projects aiming to develop infrastructure for recyclates collection & processing, and support the development of end user markets.
Enviros provides technical support to each of the ReMaDe programmes: there is a different organics focus in each ReMaDe initiative – for example:
- An organics eco-industrial site (operational demonstration and training facility) is being developed at Rainham Landfill as a partnership between Cleanaway (now Veolia), London ReMaDe, and Enviros - a covered ASP is being considered for this facility
- Composter training, along with contract, permitting and marketing support is provided by ReMaDe Essex.
- The Clean Merseyside Centre seeks to promote and support composting infrastructure development
Contacts
For further information on composting infrastructure and market development contact:
Phil Wallace
Enviros
T: 0161 874 3635
Email: compost@enviros.com
For further information on the Gore™ Covered Aerated Static Pile system contact:
Bernhard Kiehl
Solid Waste Treatment
W. L. Gore & Associates GmbH
Email; gorecover@wlgore.com
WebSite: www.gore-cover.com
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