Database of Waste Management Technologies Life

Composting 2: Composting in covered windrows

 

Description

This process is also called “aerated static pile composting”. In aerated static pile composting a windrow is formed on top of perforated pipes or a perforated floor through which air is blown or sucked. This ensures that the material remains aerobic and offers a degree of temperature control. The windrow is not turned. The process takes 8 – 20 weeks to produce compost.

Usually, each windrow is covered with breathable sheets. The sheet allows the release of the air pumped into the maturing material but retains the odours typical of the process. Also the sheet is impermeable to rainfall ensuring the compost does not get too wet from rainfall and therefore create excess leachate. The fabric is breathable to water vapour and allows the transpiration of moisture from the heap.

Process Mass Flow Diagram

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Process Photo

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Figure 4: Aerated static pile composting

Process Operational Data

Area

To operate efficiently, a composting facility must allot sufficient space to the pre-processing, processing and post-processing compost stages, the associated materials handling equipment and the manoeuvring thereof, and the aeration system. Typically, the bulk of the site (2/3 of the total area) will be occupied by the composting pad. A variety of factors determine the dimensions of the area requirement. Among them are the total volume of material to be processed, the composting period, the configuration of the windrows, nearby land uses and the existing infrastructure (EPA, 1994).

The covered piles are usually of indeterminate length, about 4m wide and 2.5 m high (Tchobanoglous, 2002). For a composting period of 8-20 weeks, a typical density of organic fraction 250 kg/m3 and typical dimension of windrows: 2.5m height, 4m width and 20m length the site area ranges from 225 to 550 m2 per tonne of feedstock per day, while Golder Associates Ltd (2009) reports 0.50-1.0 m2 per tonne of feedstock per year and McDougal et al. (2002) reports 1.45 m2 per tonne of feedstock per year.

Energy

Fuel is used for the operation of windrow turners, for materials loading and transportation, while electricity is consumed in the operation of machinery used for pre and post-treatment operations. According to van Haaren et al. (2010) total energy requirements for aerated static piles are 18.3 kWh per tonne of feedstock. This figure includes fuel (15.00 kWh/tonne) and electricity (3.3 kWh/tonne) and compares to 25.2 kWh per metric tonne of organic feedstock based on Glaub et al. (1989), while Fricke et al. (2005) determined the electricity needs for a forced-aeration composting process in 30-60 kWh per metric tonne of organic feedstock.

Water

Although water is produced during volatile solids decomposition, the heat and airflow generated during composting evaporate significant amounts of water and tend to dry the material out. This volatilization and removal of water plays a central role in heat removal and temperature control. During the active composting phase, additional water usually needs to be added in order to keep the moisture content from dropping to inhibitory levels and prevent premature drying and incomplete stabilization.

According to data from international literature water consumption is around 0.14-0.33 m3 per tonne of feedstock (Cadena et al. 2009), depending on the existence of a wet scrubber in case of in-vessel systems.

Process Environmental Indices

Air Emissions

Although during the composting process more than 100 types of gaseous compounds can be emitted (Chung, 2007), N2O, NH3, CH4 and Volatile Organic Compounds (VOCs) represent together with CO2, 99% of the total emission (Beck-Friis et al., 2000; Pagans et al., 2006; Amlinger et al., 2008).

The air emissions from composting are not straightforward to measure or to present. The gaseous emissions tend to be fugitive in nature. In addition, they can be expected to depend upon a number of inter-related factors (Arcadis et al., 2010):

  1. The nature of the input wastes, in particular the nature of the organic carbon in the components of the waste, and the nature of any organic compounds in the input wastes which may be released as the mass of material heats up;
  2. The nature of the process, and the retention time in that process, as well as the maturation period;
  3. The nature and effectiveness of the turning / airflow systems, and the frequency of turning;
  4. The regime of management of moisture in the biomass, especially in turned windrow systems;
  5. The C:N ratio of the bio waste; and
  6. The nature and effectiveness of any measures to control air pollution. Implicitly, this means that gaseous emissions from windrow facilities will be higher for some gases than they will be at enclosed facilities making use of biofilters.

CO2 emitted from composting is not fossil-derived, and therefore, it is not considered as a greenhouse gas emission (Amlinger et al., 2008), in accordance to the European Commission (Smith et al., 2001) and UNFCCC (Approved Baseline Methodology AM0025, 2005).

During storage of waste in collection containers, the composting process itself and when the compost is finished, N2O emissions might be released. Based upon Schenk (1997) and others, a total loss of 42 mg N2O-N per kg composted dry matter can be expected (from which 26.9 mg N2O during the composting process). Assuming 650 kg dry matter per tonne of compost and 42 mg N2O-N, and given the molecular relation of 44/28 for N2O-N, an emission factor of 0.043 kg N2O per tonne of compost is calculated (UNFCCC, 2005). According to Hellmann (1995), nitrous oxide emissions range from 0.012 to 0.114 kg N2O-N per tonne of basic dry mass depending on N-content of the feedstock and the composting conditions. Grontmij and IVAM (2004) propose a value of 0.101 kg N2O per tonne of organic feedstock, while Gronauer et al. (1997) estimate 0.150 kg N2O per tonne of waste. According to measurements of N2O emission from windrows presented by Amlinger et al. (2008) emissions of N2O ranged from 0.116-0.178 kg per tonne of waste to the facility, while Arcadis (2010) estimates 0.116 kg N2O per tonne of waste treated at a windrow facility.

Ammonia (NH3) emissions are determined by the quantity of ammonium ions, urea, and organically bound nitrogen. The pH value, temperature, ventilation, and the C/N-relation constitute other influencing factors. An increasing pH value, higher temperature, and/or better ventilation lead to greater emissions. High C/N relations cause NH3 emissions to diminish (Arcadis, 2010). According to Arcadis 9% of the input nitrogen is converted to ammonia and 1% to N2O, resulting in emission of 1.04 kg ΝΗ3 per tonne of waste treated. Gronauer et al. (1997) suggest that around 12% of total nitrogen escapes from the material in the form of ammonia, which corresponds to a figure of 0.53 kg ΝΗ3 per tonne of organic waste. According to Komilis and Ham (2004) NH3 emissions are 2.50 kg ΝΗ3 per tonne of yard waste feedstock.

During the composting process, aerobic conditions may not be completely reached in all areas and at all times. Pockets of anaerobic conditions – isolated areas in the composting heap where oxygen concentrations are so low that the biodegradation process turns anaerobic – may occur, so such areas may be potential emissions sources for methane (CH4) just like an unmanaged landfill is. According to Bokhorst et al. (2001) aerobic processes are replaced by anaerobic composting processes if oxygen content in compost is below 5% - 7.5%. According to Amlinger et al. (2008), CH4 emissions for open windrows range between 0.050-0.600 kg per tonne of organic waste to the facility with lowest values being indicative of well managed composting processes (Arcadis, 2010).

Relatively few studies make reference to emissions of VOCs. According to measurements of UK Environment Agency for existing facilities (2000), total VOCs emissions are 24 g per tonne of organic feedstock.

Odour is a significant impact of composting facilities. Odours are emitted from the surface of open piles, windrows, maturation piles, storage piles, and stockpiles of amendments. Typically the most problematic odorous compounds at composting facilities include ammonia, hydrogen sulphide, mercaptans, alkyl sulphides such as dimethyl sulphide and dimethyl disulphide and terpenes. These compounds are present in many composting feedstocks or are formed during the process through aerobic or anaerobic actions. Effective operational management, such as processing incoming feedstocks as soon as possible, managing the process properly, following good housekeeping practices, can help to control the formation of odours. In the case of closed piles, the windrows are entirely covered with a 12- to 18-in (30.5- to 47.7-cm) layer of wood chips, finished compost, or similar material, which absorbs objectionable odours minimizing odour problems (Tchobanoglous, 2002).

Wastewater

Leachate is the free liquid that has been in contact with compost materials and released during the composting process. Even well-managed composting operations will generate small quantities of leachate. Leachate pools are a result of poor housekeeping and may act as a breeding place for flies, mosquitoes, and odors. Leachate can also contaminate ground- and surface-water with excess nitrogen and sometimes other contaminants. For these reasons, leachate must be contained and treated. Many Composting Facilities use a concrete pad to collect and control any leachate that is produced. The primary task here is to watch the edges, catching any leachate before it leaves the pad. The simplest way to handle leachate is to collect the water and reintroduce it into the compost pile. Leachate may contain pathogens, and therefore must not be returned to material that has been through the pathogen destruction stage (EPA, 1995).

Piles left outdoors (without a roof) will be exposed to rain, which will generate leachate. Attempts must be made to minimize leachate production by diverting any surface-water runoff from the up-slope side of the piles. Another method is to shape the peak of the pile concave, so the rain water will soak into the pile rather than shed off the pile (EPA, 1995).

Excess amounts of leachate beyond the moisture needs of the composting facility can be transported to a municipal wastewater treatment plant, or treated onsite in a waste treatment plan (EPA, 1994).

According to Komilis and Ham (2004), varying amounts of leachate have reportedly been produced in MSW and garden waste composting facilities starting from 0 to approximately 0.49 m3 per tonne of feedstock, while Krogmann and Woyczechowski (2000) report 0.030 m3 per tonne of organic input to the facility for turned windrows.

Residual

The solid wastes of Composting Facilities include mainly non-compostable, inert and ferrous materials, removed during pre-processing and post-processing stages, which are disposed to landfill. According to data from existing facilities residual is up to 0.4 tonnes per tonne of feedstock.