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The Pollboy In-Vessel Composting Facility: An Overview

Introduction

Galway County Council in conjunction with Ballinasloe Town Council commissioned the building of an in-vessel composting facility at Pollboy landfill to process the domestic organic waste being produced within the administrative area of the town council. An in-vessel system was chosen for the site given the superior environmental and process control features offered by enclosed systems over conventional open windrows. The facility was designed and installed by Celtic Composting Systems Ltd, an Irish/US company that specialises in state-of-the-art composting systems. The facility went into operation in March 2002.

The Composting Process

(1) Waste receipt
Organic kitchen and garden waste is delivered every other week to Pollboy. The initial throughput of material represents the production from 1,500 homes, estimated at between 400-600 tonnes per year. Some green waste that is delivered directly to the landfill is also processed. Once weighed, the material is discharged onto the floor of the covered tipping building and large contaminants such as refuse sacks are removed. The protocol for the facility is for the immediate enclosure of the waste material within the aerated composting reactors (ACRs) on the day of arrival to avoid vermin, odour and leachate issues.

(2) Waste blending and loading
Once an initial inspection has been conducted, the material is loaded into an auger mixer to blend the waste with bulking materials to allow the material to be adjusted for moisture, nutrient ratio, porosity and microbial activity. The blended "pre-compost" is then transferred to the ACRs by conveyor.

Fig. 1. The two reactors at the Pollboy facility. The inlet and outlet air lines plus the leachate hose are illustrated.

(3) The Aerated Composting Reactors
The reactors are designed to optimise the initial high-rate composting phase through the enclosure of the composting biomass. On a 14 day cycle, the reactors at the Pollboy facility are each rated at approximately 1-1.3 tonnes of pre-compost capacity per day (365-475 tonnes per unit per year; Fig. 1). Air is delivered to the reactors through a perforated floor with exhaust gases being drawn from the void volume in the roof of the unit. The enclosure of the fresh material at the start of the process offers a number of environmental advantages.

(a) Vermin are excluded during the initial phase while the material has a high food value.
(b) All leachate is captured and directed to the leachate collection system.
(c) All exhaust gases are directed to a biofilter that removes off-odours.
(d) The reactors have a neat and tidy appearance
(e) Wind blown litter is eliminated.
(f) Rain is excluded and therefore water-logging of the biomass is avoided.

(4) Process control
The rate of air-flow through the reactors is dictated by temperature measurements from within the compost pile that maintain composting conditions at an optimum during this initial 14 day high rate cycle. This air-flow operates in two modes.
(a) Oxygenation mode supplies a sufficient air-flow to support aerobic respiration in the reactors.
(b) Cool-down mode supplies air as a heat exchanger to cool the compost pile when temperatures rise above optimum.

This "temperature feed-back" mechanism is automatically controlled by a programmable logic controller (PLC) linked to proprietary CCS software. The software continuously logs temperature within the reactors to document that each batch has been pasteurised for quality control purposes. Automated CO2/O2 feed-back analysis is also available to further enhance aerobic activity.

(5) Curing
At the end of the initial 14 day cycle the compost has been pasteurised and has lost most of its food value. It is now safe to unload it into the curing bunker. The curing bunker is an engineered composting system designed by CH2MHILL on behalf of CCS. CH2MHILL is a world leader in compost engineering and the aerated static pile (ASP) system can be used directly as a stand-alone composting technology for lower priority materials such as green waste. However, for the purposes of food waste, it is used as a secondary composting and curing system in conjunction with the reactors. The compost is discharged from the reactors onto perforated pipes that draw air under vacuum from the atmosphere through the piles and this air is re-pressurised and forced through another biofilter to remove any residual odours. In addition, all leachate and condensate is captured and directed to the leachate interceptor system. The material is typically maintained in the curing area for eight to 10 weeks prior to screening. Temperatures are routinely monitored and the air-flow to each curing cell is subsequently regulated by valves on the manifold pipe to optimise aeration and the temperature regime, while avoiding over-aeration which can dry out the piles and waste electricity. In this regard, the Pollboy facility is currently operating from a single-phase domestic power supply.

Fig. 2. The C:N curing system illustrating the compost piles, aeration lances and the biofilter at the far end of the bunker

(6) Screening
At the end of the composting process, the compost is screened using a trommel. The resultant material has a fine structure and is currently being beneficially re-used as a soil conditioner and surface mulch in the Ballinasloe area.

Fig. 3. The result material is a compost which can be used as a soil improver for shrubs and plants

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The author acknowledges the kind permission of Mr Tom Kavanagh, Director of Environment & Conservation Services, Galway County Council, to publish this article.

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