Environmental Engineering Reference
In-Depth Information
methanogenic bacteria at concentrations of 100-150mg S l
1
(Speece, 1983).
A further effect is that soluble sulphides can form complexes with trace
elements to form insoluble sulphides; and for this reason iron is often added
to the digester in the form of iron salts to reduce sulphur toxicity and
improve microelement availability. Hydrogen sulphide can also be removed
by micro-oxidation and precipitation of elemental sulphur. As H
2
Sisof
concern to operators because of its corrosiveness, odour and toxicity to
humans, H
2
S removal may also be regarded as a process optimisation
parameter.
6.5.3 Micronutrients
Micronutrients are essential to methanogenic consortia as they form part of
the coenzymes used in many of the reactions leading to methane formation.
If their supply is restricted, the capacity of the population to synthesise
enzymes is likewise restricted, which in turn will compromise the metabolic
capacity of the digester. Trace elements not only have to be present, but also
need to be in a bio-available form. Any inadequacy either in trace elements
or in their bio-availability will result in lower maximum loading rates and/or
less efficient treatment. The build-up and persistence of VFA in digestate
may be indicative of nutrient deficiency, but if bio-availability and the
presence of nutrients is satisfactory then toxicity is an alternative that should
be investigated.
Although iron was known to be stimulatory to AD, it was not until the
1980s that trace elements were found to be the missing factors needed to
maintain activity in digesters fed on pure culture media. Nickel was the first
identified, followed by cobalt, and the practical benefits of this were soon
realised: in a survey of AD plants treating municipal wastewater biosolids,
Speece (1988) noted that 8 out of 30 digestates tested showed increased rates
of acetate utilisation and methane production when supplemented with Fe,
Ni and Co. Since then, studies have consistently shown that the addition of
trace elements can be beneficial to the digestion process for a wide range of
feedstocks and digester types. Likewise, trace elements have been identified
that are important in methanogenic metabolism as well as necessary for the
process of acetogenesis. A summary of where these elements are used in
different metabolic pathways is given in Table 6.2.
It is difficult to provide a trace element dosing guide to satisfy every
eventuality, because trace elements form insoluble precipitates with sulphide,
carbonate and phosphate. The elements may be kept soluble by chelation
(combinationwithanothermolecule), butmaybind stronglywith the chelating
agent forming complexes that again render them unavailable. Overdosing to
try and overcome these limitations is not a good policy, as certain trace
elements may be stimulatory at low concentrations but toxic at higher ones.
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