Agricultural-Based Food Production Geoenvironment Stressors - Sustainable Practices in Geoenvironmental Engineering - page 199

6.5.3.3 Wetlands

The liquid portion of efluents from farm operations must usually be treated because of
their high nutrient and organic contents. Since the wastewater contains a higher concen-
tration of soluble ammonia than the initial feed, land spreading is a viable option since
this allows for easier uptake by plants. Algal ponds or constructed wetlands may also be
used to improve water quality to allow water reuse. Constructed wetlands are operated as
subsurface low or free surface low (Mulligan, 2002). Nutrients are removed by the plants
growing in the wetlands. Subsurface systems are not as eficient for nutrient removal.
Hammer et al. (1993) has reported on a two-cell surface system used for a 500-swine opera-
tion to reduce 90 to 36 kg BOD/day in a 3600-m 2 wetland. Nitrogen loading rates should be
from 3 to 10 kg/ha-day, and ammonia concentrations in the inluents should not be higher
than 100 to 200 mg/L. In general, N, P, and solids reductions should be greater than 50%
and BOD greater than 60% if the wetlands are not overloaded. Wetlands offer a method of
enhancing biodiversity while treating liquid efluent discharges.

6.5.3.4 Integrated Manure Treatment

A comparison of manure management methods is shown in Table 6.4. The choice of the most
appropriate method for management of manure must include minimization of emissions
and other impacts on the environment such as decreased water, groundwater, and soil qual-
ity. Dalemo et al. (1998) utilized a simulation model (ORWARE) for the calculation of energy
and nutrient lows from soil and liquid organic wastes from restaurants. Composting and
anaerobic digestion were compared. A life cycle approach was used that included the pro-
cesses and soil emissions from the product. Soil emissions were in the form of N 2 O produc-
tion. Since the content of ammonia is lower in the compost, emissions of ammonia in the soil
are also lower in comparison to anaerobic digestion residues where the organic nitrogen is
mineralized. In respect to concerns for eutrophication, (a) emissions are mainly in the form
of nitrate leaching from the soil, (b) ammonia is released during composting, and (c) NO x is
released during combustion of biogas for electricity generation. In short, composting pro-
duces ammonia emissions during the composting process, in addition to NO x and SO x .
In the various models compared by Hansen et al. (2006), such as DST (Decision Support
Tool, USA), IWM (Integrated Waste Management Tool, United Kingdom), IFEU (Germany),

TABLE 6.4
Comparison of the Sustainability of Manure Treatment Methods

Treatment Energy Emissions Products Costs

Land
spreading

None produced, energy
for spreading

Ammonia, methane,
N 2 O, odor, leaching of
N, P, bacteria

Fertilizer Low

Composting Energy for mixing and

mechanical separation,
heat generation

Carbon dioxide,
ammonia losses higher
than anaerobic

Compost for soil
improvement

Depends on
sophistication

Anaerobic
digestor

Energy produced in form
of methane, heat
required for the digestor

High BOD liquid
efluent

Methane for
electricity or
energy

Depends on
sophistication

Anaerobic
lagoon

Energy produced in
form of methane

Odors, N 2 O, and
methane, if not
covered well

Methane for
electricity or
energy

Low

Source: Adapted from Dalemo, M. et al., Resources, Conservation and Recycling 24, 363-381, 1998.