Domestic sludge treatment

Sludge and other biodegradable organic wastes hold enormous potential for energy generation. The amount of energy that is released upon oxidation of organic matter is around 14.5MJ (kg O2)-1 (Blackburn and Cheng, 2005). The amount of energy released depends on the biodegradability of the organic matter under certain conditions. The most commonly used methods for the treatment of biological wastes will be discussed below.

Landfills

Disposal of waste at landfill sites should be carried out in a proper way to avoid unwanted CH4 emission due to decomposition (see Chapter 11). The recovery of landfill gases is now a common way to reduce unwanted emissions (IPCC, 2006b). It is difficult to assess the effect of sludge disposal on CH4 emissions. A generally accepted way is to use the first-order decay method that assumes a slow decay of organic matter. Depending on the conditions (especially temperature), CH4 generation rate constants for the decay of sewage sludge in landfills range from 0.05 to 0.1 year-1 in dry conditions, and from 0.1 to 0.7 year-1 in moist and wet conditions (IPCC, 2006b).

Digestion

The emission due to digestion of waste sludge can be calculated with the following equation:

CH4 emission, domestic = ^ M x EF•) X 103 - R (14)

where M; = mass of organic waste treated by biological treatment type i; EF is emission factor for treatment i; i refers to composting or digestion; and R = total amount of CH4 recovered (IPCC, 2006a).

The CH4 emission factor is the main determining factor for the CH4 emission and this is highly variable. It is determined by the nature of the waste and supporting material (if any), the temperature and moisture content, and the energy needed for aeration (in the case of composting) (Table 10.5) (IPCC, 2006d). Emissions from anaerobic digestors due to unintentional leakages of CH4 are estimated to be between 0 and 10 per cent (5 per cent is the default value).

Table 10.5 The methane emission factors (EF) of anaerobic digestion at a biogas facility and during composting

Dry weight basis

Wet weight basis

(moisture content 60%)

Anaerobic digestion

2 (0-20)

1 (0-8)

Composting

10 (0.08-20)

4 (0.03-8)

Note: There is an assumption of 25-50 per cent dissolved organic carbon (DOC) in dry matter, and 60 per cent moisture in wet waste.

Source: Data taken from IPCC (2006a)

Note: There is an assumption of 25-50 per cent dissolved organic carbon (DOC) in dry matter, and 60 per cent moisture in wet waste.

Source: Data taken from IPCC (2006a)

Typical biochemical CH4 potential data are given in Table 10.6. As expected, the nature of the sludge largely determines the BMP. In some cases the CH4 production can be further increased by thermal, chemical, physical and/or thermo-chemical pretreatment. In this way more CH4 may be generated for further use as an energy source, with the concurrent effect that undesired CH4 emissions after the digestion process are mitigated. An example of the effect of thermo-chemical pretreatment on CH4 production is given by Kim et al (2003). They reported a 34 per cent increase in CH4 production during digestion of waste activated sludge after thermo-chemical treatment at 121°C for 30 minutes at pH 12. Likewise, Tanaka et al (1997) observed a 27 per cent increase in CH4 production after thermo-chemical pretreatment of the sludge.

Composting

The CH4 emissions during composting of waste sludge can be calculated with the equation given above for digestion. Although composting is an aerobic

Table 10.6 Typical BMPs for different sludge types and other biowaste

Sludge type m3 CH4 tonne-1 OS

Reference

Primary sludge (domestic) 600

Primary sludge (industrial) 300

Domestic sludge 172

Secondary sludge (domestic) 200-350

Flotation sludge 690

Source-separated municipal biowaste 400

Han and Dague (1997) Braun (2007) Tanaka etal (1997) Braun (2007) Braun (2007) Braun (2007)

Note: OS = organic sludge process, CH4 may be formed in the anaerobic zones of the heap. Significant methane emissions are very unlikely to occur during composting because any CH4 formed will be oxidized in the aerobic zones of the compost by methanotrophic bacteria. Total CH4 emissions from composting will be limited to less than 1 per cent of the initial carbon content of the sludge (or any other material to be composted) (IPCC, 2006c).

Other methods

Incineration of solid wastes - sludge remains after composting for example -is generally carried out in controlled facilities. Emission factors are difficult to assign. Also, the effect of land application on emissions is difficult to assess.

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