TECHNICALLY SPEAKING
Don’t pour that away
Wastewater used to considered just that, but not any more. With rising costs, there is increasing
interest in recovering the potential energy – and putting it to good use. Frank Rogalla explains
W
hat is wastewater worth? While
municipal effluents have typically
been considered a waste product
and management has focused on disposal and
Digester Gas
minimisation of residuals, raw wastewater 3
675 m /h Engine Generator
contains potential energy with its organic content,
15Gj/h 35% n
amounting to almost 300 kWh/person/yr. With
rising energy costs, there is increasing interest in
recovering this inherently available energy.
Anaerobic digestion, primarily of the solids
generated in wastewater processing, produces Electricity
methane, which has historically been used as a
1.5 MWh/h
source of energy for the treatment process by
or 5.3 Gj/h
Anaerobic
converting it to power production with engine
Digesters
generators, microturbines, and fuel cells. Feed Solids Digested
45% VSr
Improvements in gas cleaning technologies
45 tonnes/d Solids
3
1m gas/kg VSr
and increasing power costs have made this
80% VS Excess Heat
practice more cost effective. In addition,
35 Gj/h 6 Gj/h
processes that improve digestion or increase gas
production, such as co-digestion of high energy
substrates or digester pre-treatment, are being biosolids digestion. Biosolids are a significant in a follow-on combustion process.
widely implemented. The energy in the solids source of energy, typically containing 6.5 kwh/kg Based on the concept of conservation of
can be recovered through combustion processes volatile solids, which is similar to the energy energy, the total amount of energy liberated
as well, such as incineration with waste heat content of low grade coal and could be through different pathways is theoretically the
recovery and subsequent power generation. A used as a fuel source for power plants or same; however, practical limitations on process
number of “emerging” technologies, such as industrial processes. efficiencies and energy losses means that net
supercritical water oxidation, pyrolysis, and Heat dried biosolids are commonly used for energy output varies for different technologies.
gasification of biosolids, while technically cement kilns in Switzerland and Germany, fossil
feasible, are being investigated to determine fuel and have recently begun being used for Biogas Options
cost effectiveness. cement production in North America. Dried Anaerobic digestion has long been used as a
While wastewater contains energy, the organic biosolids are suitable for co-burning at cyclone method to reduce mass and to improve the
material is also the source of “pollution” from the furnace or pulverised coal burning plants, with aesthetic qualities of biosolids, as well as using
standpoint that, if discharged to receiving waters, few modifications to the power plants. the biogas generated through the digestion
it decomposes and consumes receiving stream Permit conditions are expected to be process as a source of energy, for both process
dissolved oxygen resulting in stress, or death, in unaffected with biosolids to fuel ratios of 10 to heating and for conversion to electricity. Europe
the ecosystem. By contrast, approximately 20% (EPRI, 1998). Co-burning at biosolids to has been a leader in implementing electric
50kWh/yr/person, or 17% of influent energy fuel ratios of 10 to 20% as fuel value have been generation from biogas, and an estimated two
value, is needed to operate most conventional evaluated for a number of facilities in the US, but thirds of the biosolids in the United Kingdom are
wastewater treatment plants, has not yet been implemented. digested with subsequent power generation. In
where the organic material is normally degraded Mostly, biosolids energy is recovered through the United States, still less than 20% of publicly
with biological treatment systems which use processes that break the carbon bonds in the owned treatment works (POTWs) with anaerobic
large amounts of purchased energy to supply solids. Two pathways can be used to convert digestion recover biogas energy, corresponding to
the oxygen. the inherent energy of the biosolids into a about one third of the biogas generated. Only
Emanating from these conventional treatment useable form: regions with high electric rates and favourable
processes are treatment residues (biosolids) regulatory environments, such as California, have
which still contain significant energy value, and ■ Thermal processing (hereafter referred to as rates of recovery similar to Europe, for instance
conventional anaerobic digestion allows recovery “Combustion”), such as incineration, where the 60% of the plants larger than 38 ML/d use their
of approximately 11% of the influent energy via biosolids can be oxidised to carbon dioxide biogas for electricity generation.
electrical cogeneration operating on the (CO
2
). An example of one project evaluation with a
produced methane gas. ■ Biochemical treatment that generates biogas, heat balance for biogas utilisation in a treatment
In other words, more than half the energy such as anaerobic digestion, to convert the plant with a capacity of half a million people is
required to operate a traditional wastewater energy to methane, followed by thermal shown on Figure 1. On average case, usable
treatment plant can be recovered with anaerobic processing, which can then be oxidised to CO
2
energy, including heat, is around 6 GJ/tonne
38 Water & Wastewater Treatment September 2009
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