Emissions from manure

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There are several ways to treat manure that is produced on a farm (Figure 10.4). Emissions from manure related to storage, treatment and handling are also a result of anaerobic fermentation processes, which partially take place due to the presence of enteric bacteria that are excreted into the manure by the animals. On-site or central digestion is a recognized pathway for the treatment

Figure 10.2 Maximum solubility of methane at different temperatures

Source: Data for Henry's constant taken from Metcalf & Eddy Inc. (1991)

Figure 10.2 Maximum solubility of methane at different temperatures

Source: Data for Henry's constant taken from Metcalf & Eddy Inc. (1991)

Methane capacity exceeded

Volatile Fatty Acid increases pH

decreases

Poor buffering capacity

Methanogenic toxicity increasing pH

decreases

Unionized VFA increasing

Unionized VFA increasing

Figure 10.3 Problems related to overloading of anaerobic treatment systems

Source: Redrawn from van Lier et al (2008)

of manure for producing biogas and reducing CH4 emissions from storage. The biogas production in digesters depends on the nature of the manure (Table 10.1) and on process parameters that have been discussed elsewhere in this chapter. The digestion of manure and factors related to its success have been reviewed extensively (see van Velsen, 1981; Zeeman, 1991; El-Mashad, 2003). Similar to other anaerobic processes, the biogas that is produced contains CH4 and CO2. The CH4 emission from manure is calculated by multiplying the amount of manure per animal category and/or country or region with an emission factor. This emission factor depends on the fraction of organic matter, the gross CH4 production potential and a conversion factor representing the percentage of the CH4 potential that is actually realized in a specific manure handling system (VROM, 2008b).

Enhancing the production of CH4 and capturing the biogas is an abatement strategy, generally referred to as anaerobic digestion. The captured gas can then be used as fuel for heating, lighting, transport or production of electricity or can be burned without beneficial use (flaring). The positive greenhouse gas emission effect is based on the conversion of CH4 (GWP of 21-25) to CO2 and the replacement of electricity from fossil fuels by renewable energy in the form of CH4.

The emissions reduction potential of anaerobic digestion in manure treatment and storage systems depends on the emissions from the reference system, and ranges for liquid manure systems from 50 per cent in cool climates to 75 per cent in hot climates (Steinfeld et al, 2006). An emission reduction of 85 per cent for large-scale CSTR and plug-flow reactors has been achieved in developed countries, with a reduction of around 50 per cent reported for small-scale reactors in developing countries (US EPA, 2006).

Methane production is often increased by so-called co-digestion of manure with other biomass. This can be dedicated crops, crop residues, residues and leftovers from the food industry, or biomass from nature reserve areas (refer to Table 10.1 for examples). Digestion of manure by co-digestion is considered to shorten the length of time in pre-storage at farms. However, because of the relatively low biochemical methane potential (BMP) of manure (Table 10.1) compared to, for example, maize (around 0.3m3 CH4 tonne-1), and the lower organic matter content of manure compared to maize (Amon et al, 2007) and other co-substrates, the effect of shorter storage times may be offset. In The Netherlands, the amount of co-substrate that is permitted while still treating the digestate as 'fertilizer of animal origin' is 50 per cent on a volumetric basis.

Total greenhouse gas emission reduction for the whole biogas production chain, including co-digestion of biomass, is difficult to establish as emissions of alternative processes and the attribution of emissions in proceeding processes are debatable.

Post-storage of digestate of manure is gaining increasing interest as a possible source of unwanted CH4 emissions. An inventory of 15 different digestates showed an average residual gas formation of 5m3 CH4 tonne-1 (range 1-10m3 CH4 tonne-1 digestate), for central manure systems, which may be 8

Figure 10.4 Different ways to handle manure

per cent of the total CH4 mitigation potential (I. Bisschops, personal communication, 2009). Post-digestion in, for example, gas-tight covered storage is therefore promoted to optimize CH4 production and to avoid unwanted CH4 emissions. The BMP is often not achieved in digestors due to insufficient hydrolysis. Angelidaki et al (2005, 2006) analysed the potential CH4 production/emission of digestates of central manure digestion systems. In the case of central manure digestion, the central storage time of digestates is generally short due to high land prices, while for on-farm storage sites, incentives for biogas collection are often lacking (Angelidaki et al, 2005).

Existing technology offers several possibilities to mitigate unwanted CH4

emissions:

The storage temperature of the slurry can significantly influence the emissions. Storage outside at lower ambient temperatures, depending on the climate conditions, can reduce the emission compared to those from animal housing. With active (deep) cooling, the emission of CH4 can be further reduced but only with high energy costs and a risk of increased CO2 emissions associated with electricity production (Sommer et al, 2004). Frequent removal of the slurry from animal housing has the advantage of reducing the emitting surface and concentrating the slurry. It can also make other abatement options - such as covering, filtering, cooling or air treatment - more effective. The storage areas should be completely emptied to avoid inoculation of fresh manure with active microorganisms (Zeeman, 1994).

Gas-tight covering of storage is a relative easy and cost-effective method to reduce emissions of both CH4 and ammonia.

Solid separation leads to removal of much of the organic matter required for CH4 producing processes, and thus lowers the emissions of CH4 from the remaining liquid part.

Aerobic treatment of stored slurry can reduce CH4 production by inhibiting microbial methanogenesis, but is associated with high energy consumption and a risk of elevated N2O emissions.

Composting of slurries together with other organic solids, or composting of solid farmyard manure, can reduce the emission of CH4 but may also increase emissions of N2O.

Possibilities of treatment of air from animal houses or manure storages with biofilters (Melse, 2003). A CH4 removal efficiency of 85 per cent was achieved in a test system but, due to low CH4 concentrations in exhaust air of housing and storage, the required size of the biofilter to treat the air from a 1000m3 storage area would be 20-80m3. The costs of such a strategy were calculated at EU€100-500 per tonne CO2-eq emission reduction.

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