Environmental Engineering Reference
In-Depth Information
losses by runoff. Others such as Delschen (1999), however, found that although the addi-
tion of 2.6 tonnes of composted manure led to higher accumulation rates of SOM than
untreated manure, the amounts were approximately the same if the 40% to 60% carbon
loss during composting is taken into account.
Various composting systems have been described by Mulligan (2002). Production of
compost for soil conditioning leads to a more stable product with practically no odor. The
more simple processes may also lead to increased ammonia emissions. The selected bulk-
ing material can inluence ammonia emissions. Emissions of methane and nitrous oxide
depend on the aeration rates. The beneits of compost for soil conditioning are well known.
6.5.3.2 Anaerobic Digestion
The organic content of manures can be treated by anaerobic digestion. The methane pro-
duced can be used for fuel or electricity production. Treatment of the manure by anaerobic
digestion can signiicantly reduce the impact on water resources. Greenhouse gas (car-
bon dioxide, in particular) emissions are reduced and the products have improved fertil-
izer capability. Nitrogen and phosphorus availability for crops are increased, reducing
chemical fertilizer requirements. A comparison of the mineralization of N of anaerobically
stored manure with composted ruminant manure showed that anaerobic manure loses
less nitrogen than during composting and therefore is a better source of inorganic N for
fertilizer (Thomsen and Olesen, 2000). Only some of the nitrogen in anaerobic residues is
organically bound, whereas most of the nitrogen in compost is in this form. It has been
estimated that digestion can reduce greenhouse emissions by 1.4 kg of carbon dioxide per
kg of volatile solids (VS) in manure. It has been reported that anaerobic treatment reduces
N
2
O emissions by more than 50% due to VS reduction after spring application onto soil, in
comparison to untreated manure (Sommer et al., 2004). Although pathogens are reduced,
the inactivation may only be about 1 to 2 log at 30°C (Burton and Turner, 2003).
In 2010 in Germany, there were approximately 1000 anaerobic plants using agricultural
substrates (Biogas an all-rounder, 2013). The combination of government and environmen-
tal factors has led to the substantial growth in the numbers used on animal farms. The EU
Waste Framework Directive requires that all member states recycle 50% of their municipal
waste by 2020. Although all types of manure are digestible, cow manure is more dificult to
digest because of the higher iber content, as opposed to pig and poultry manures. Typically,
yields of methane are in the order of 290 L/kg of volatile solids (VS) for pig manure and 210
L of methane per kg of cattle manure (Burton and Turner, 2003). Co-substrates are often used
to enhance the carbon and nutrient contents. These substrates include fodder beet and other
green wastes. Cow manure in particular is well suited to co-digestion as it already contains
high iber content. The nitrogen content of the manure serves as pH buffering and as a con-
tinuous inoculum. The co-substrates also increase methane yields (Mulligan, 2002).
Lagoons have lower capital and operating costs than digestors. Organic matter is reduced
while nitrogen and phosphorus remain in the end product. The use of lagoons is more fre-
quent in warmer climates. Proper sizing and management of lagoons are required to ensure
odor control. Loading rates of 60 to 90 g of VS/m
3
of lagoon/day have been suggested by
the Natural Resources Conservations Service in the North East of the United States. Since
methane is a greenhouse gas, gas collection should be practiced to avoid release into the
atmosphere. Odors may also be a problem. Floating covers are becoming more popular.
Temperatures of 35°C are required to maintain optimal biogas production. Heating systems
may be required, particularly in the winter. Pathogen reduction in lagoons is also minimal
(Burton and Turner, 2003).
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