Sewage Sludge Treatment and Disposal Explained
Sewage sludge is a by-product of the municipal wastewater treatment
process which purifies wastewater before it is released into local
waterways. It is the solid, semi-solid or liquid residue generated during
the treatment of domestic sewage, and is in fact an amalgamation of all
the liquid wastes from society after aerobic biological treatment.
Sewage sludge must be periodically wasted, or in other words, removed,
from a treatment works so as to not cause an excessive biomass content in
the system, or result in a pass through of sewage sludge to rivers or
other surface waters.
Regulations have been developed in all the developed countries to ensure
that the public health and the environment are protected when sewage
sludge is disposed of by each of the following accepted methods:-
(1) application to the land as soil conditioner or fertilizer,
(2) disposal on land by placing it in a surface disposal site (not
permitted in the EU),
(3) placing it in a municipal solid waste landfill unit (no longer
permitted in the EU)
(5) anaerobic digestion.
Sewage sludge is produced from our wastewater plants in huge quantities
day after day, year after year. Municipalities find themselves under
never-ending pressure to get rid of the quantities produced.
Spreading the sludge to land is the cheapest solution, however, any wash-
off during rain into watercourses is highly polluting and there are
health concerns. Even if those are dealt with by, for example, injecting
the sludge into the soil below the surface, there remains the danger of a
slow and dangerous build-up of certain heavy metals in the injected
Surface disposal in drying beds needs a dry hot climate, and incurs
health risks and potential for flies and odors. The resulting material is
then still very often spread on land, with similar heavy-mtelas build-up
worries as in (1.). Also, before the sludge dries and solidifies it may
cause pollution, however, this is a reasonably economic method.
The sludge may of course be simply placed in landfill, and this is again,
not particularly expensive, or damaging as long as only small quantities
are placed compared with much larger volumes of solid (usually domestic
solid waste). However, this is now not permitted at all in many
In the US sewage sludge is typically treated and then either spread on
land or is disposed in an approved landfill. It, as we have already
indicated, typically has high levels of heavy metals and pathogens.
The sludge does have to be transformed into biosolids before it can be
disposed, and this is achieved using a number of complex treatments such
as digestion, thickening, dewatering, drying, and lime stabilization.
Sewage sludge is classified into three categories: unrestricted,
restricted and unsuitable according to its potential toxicity.
Sewage sludge is not suitable to apply on areas near ponds, lakes,
rivers, and streams without appropriate buffer areas and it is not
suitable to apply near wetlands and marshes without a permit (EPA, 1995).
Steep areas with sharp relief and undesirable geology and soil conditions
are not suitable areas to apply sewage sludge (EPA, 1995).
Sewage sludge is known to contain heavy metals, PAH's and other organic
micropollutants, and pathogens like the spores of Cryptosporidium and
Giardia. Apart from substantial health risks for humans sewage, sludge
application may result in contamination of the environment.
Yet sewage sludge can be an excellent fertilizer so long as it is subject
to adequate treatment. The current practice of reusing sewage sludge as
an agricultural fertilizer has been carried out in the UK since the 1950s
but is under threat due to concerns largely of metals build-up in soils,
and has been subject to extensive research as a result.
Finally, though these is the anaerobic digestion process which can be
used and produces a large net surplus of methane biogas, and is a
renewable energy source, replacing or eliminating the use of erath's
dwindling fossil fuels.
Digestion (fermentation) during AD s usually carried out in an acid phase
digester under mesophilic, 15 degrees to 45 degrees C., or can also be
treated in thermophilic conditions. Thermophilic conditions exists
between 45 degrees to 70 degrees C., and a detention time of about 1 hour
to 5 days is necessary.
Generally, Mesophilic conditions are preferred when the organic waste is
municipal solid waste. Anaerobic reactors are much like any other life
form, if nutrient levels are too little or too much, they will
underperform. But, current developments especially rising fuel prices are
making Anaerobic digestion far more popular than it ever used to be.
Anaerobic digestion source which is also now, with rising fuel costs,
plausibly also profitable. Steve Evans has also written for the top web
site the United Kingdom Anaerobic Digestion blog