Process Issues
At the end of the process, the water itself may be suitable for
release but, com-monly, there can be difficulty in finding suitable outlets for
the concentrated sewage sludge produced. Spreading this to land has been one
solution which has been successfully applied in some areas, as a useful
fertiliser substitute on agricultural or amenity land. Anaerobic digestion,
which is described more fully in the context of waste management, has also been
used as a means of sludge treatment. The use of this biotechnology has brought
important benefits to the energy balance of many sewage treatment works, since
sludge is readily biodegradable under this regime and generates sizeable
quantities of methane gas, which can be burnt to provide onsite electricity.
At the same time, water
resources are coming under increasing pressure, either through natural climatic
scarcity in many of the hotter countries of the world, or through increasing
industrialisation and consumer demand, or both. This clearly makes the
efficient recycling of water from municipal works of considerable importance to
both business and domestic users.
Though in many respects the
technology base of treatment has moved on, the underlying microbiology has
remained fundamentally unchanged and this has major implications, in this
context. In essence, the biological players and processes involved are little
modified from what would be found in nature in any aquatic system which had
become effectively overloaded with biodegrad-able material. In this way, a
microcosmic ecological succession is established, with each organism, or group,
in turn providing separate, but integrated, steps within the overall treatment
process. Hence, heterotrophic bacteria metabolise the organic inclusions within
the wastewater; carbon dioxide, ammonia and water being the main byproducts of
this activity. Inevitably, increased demand leads to an operational decrease in
dissolved oxygen availability, which would lead to the establishment of
functionally anaerobic conditions in the absence of external artificial
aeration, hence the design of typical secondary treatments. Ciliate proto-zoans
feed on the bacterial biomass produced in this way and nitrifying microbes
convert ammonia first to nitrites and thence to nitrates, which form the
nitro-gen source for algal growth. Though the role of algae in specifically
engineered, plant-based monoculture systems set up to reduce the nitrogen component
of wastewaters is discussed more fully in the next, it is interesting to note,
in passing, their relevance to a ‘traditional’ effluent treatment system.
One of the inevitable consequences of the functional ecosystem
basis underly-ing sewage treatment plants is their relative inability to cope
with toxic chemicals which may often feature in certain kinds of industrial
wastewaters. In particu-lar, metabolic poisons, xenobiotics and bactericidal
disinfectants may arrive as components of incoming effluents and can prove of
considerable challenge to the resident microbes, if arriving in sufficient
concentration. This is a fact often borne out in practice. In 2001,
considerable disruption was reported as a result of large quantities of
agricultural disinfectant entering certain sewage works as a consequence of the
UK’s foot and mouth disease outbreak. A number of poten-tial consequences arise
from such events. The most obvious is that they kill off all or part of the
biological systems in the treatment facility. However, depen-dent on the nature
of the substances, in microbially sublethal concentrations, they may either
become chemically bound to either the biomass or the substrate, or be subject
to incomplete biodegradation. The effective outcome of this is that the degree
of contaminant removal achievable becomes uncertain and less easily controlled.
Partial mineralisation of toxic substances is a particular con-cern, often
leading to the accumulation of intermediate metabolites in the treated
wastewater, which may represent the production of a greater biological threat.
The incomplete metabolism of these chemicals under aerobic conditions
typi-cally results in oxidised intermediary forms which, though less
intrinsically toxic than their parent molecules, are often more mobile within
the environment. In addition, when the treatment efficiency is subject to
monitoring, as intermedi-ate metabolites, these substances may not be picked up
by standard analytical techniques, which may result in an unfairly high measure
of pollutant removal being obtained.
Moreover, the extension of sewage treatment facilities to
ameliorate trade efflu-ents also has implications for the management of true
sewage sludge. It is not economically viable to develop processing regimes
which do not lead to the concentration of toxic contaminants within the derived
sludge. This was shown to be a particular problem for plants using the
activated sludge process, which relies on a high aeration rate for pollutant
removal, which proceeds by making use of biotransformation, air stripping and
adsorption onto the biomass. Adsorp-tion of toxic inorganic substances like
heavy metals, or structurally complex organic ones, onto the resident biomass,
poses a problem when the microbial excess is removed from the bioreactor,
particularly since dewatering activities applied to the extracted sludge can,
in addition, catalyse a variety of chemical transformations. Accordingly,
sewage sludge disposal will always require careful consideration if the
significant levels of these chemicals are not subsequently to cause
environmental pollution themselves.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.