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Wastewater
Our Essential
Guide To Wastewater Treatment, Management & Solutions
Aquatech
Wastewater
treatment, collection and discharge are essential to protect human health, the
environment and surrounding water quality.
Before it can
be treated, wastewater needs to be collected from sewer networks servicing
homes, municipal, commercial and industrial premises, including rainwater
run-off from roads and other impermeable surfaces.
Wastewater
treatment and industrial wastewater treatment are evolving.
Historically it
was designed to clean up wastewater before a cleaned-up effluent could be
discharged safely into the surround area.
Today,
wastewater is being seen a valuable resource to generate: energy, nutrients and
water for irrigation, industrial and even drinking purposes.
This article
provides everything you need to know about the different treatment stages and
technologies involved in wastewater treatment.
Wastewater: creating sustainable value
Why is wastewater treatment important?
Today, around
80% of all wastewater is discharged into the world's waterways where it creates
health, environmental and climate-related hazards, according to the IWA.
Estimates
suggest wastewater treatment capacity is currently 70% of the generated wastewater
in high-income countries, and only 8% in low-income countries.
Furthermore,
urbanisation further exacerbates this challenge with increasing wastewater
generation, while at the same time using more of Earth's dwindling resources,
according to the IWA.
The discharge
of untreated effluent in water bodies does not only lead to eutrophication and
human health risks, it also contributes significantly to Greenhouse Gas (GHG)
emissions in the form of nitrous oxide and methane.
Emissions from
untreated sewage represents three times the emissions of conventional
wastewater treatment.
The emissions
from untreated sewage can represent a significant percentage of cities' global
emissions, even when treatment coverage is still poor as in many emerging
cities.
Wastewater
management and adequate sewer systems play important roles in sanitation and
disease prevention.
It is vital to
develop a system to manage community wastewater and sewage.
Otherwise,
wastewater can contaminate the local environment and drinking water supply,
thereby increasing the risk of disease transmission.
Global access
to safe water, adequate sanitation, and proper hygiene education can reduce
illness and death from disease, leading to improved health, poverty reduction,
and socio-economic development.
However, in
many countries, proper wastewater management is not practiced due to lack of
resources, investment, infrastructure, available technology, and space.
Many countries
are challenged to provide these basic necessities to their populations, leaving
people at risk for water, sanitation, and hygiene (WASH)-related diseases.
In the 2030
Agenda for Sustainable Development, Goal 6 aims to guarantee sustainable
management of, and access to, water and sanitation for all by 2030.
However, in
2015, three in ten people (2.1 billion) did not have access to safe drinking
water and 4.5 billion people, or six in ten, had no safely managed sanitation
facilities.
As well as
safeguarding human health and environmental protection, modern wastewater
treatment is helping to identify ways to create value from the materials,
energy and water that is embedded in wastewater streams.
What is wastewater treatment?
Wastewater
treatment involves the collection of wastewater often by thousands of
kilometres of sewer pipes.
The size of
collection systems varies depending on the region and country they serve. For
example, the largest collection systems in the UK are linked to around 9000
wastewater treatment plants.
In some
countries and areas, rainwater from roofs, roads and pavements is collected in
a separate system, called a surface water sewer, which goes straight into
river.
Alternatively,
wastewater and surface water are mixed together in combined sewers before
wastewater treatment.
A recent
development in the UK using fibre sensing cables is evaluating the measurement
of flow, depth, temperature and structural integrity every five metres along a
sewer pipe.
Once wastewater
reaches treatment plants, four stages are involved, including:
· Preliminary treatment (pre treatment) – to remove grit and
gravel and screening of large solids.
· Primary treatment – to settle larger suspended, generally
organic, matter.
· Secondary treatment – to biologically break down and reduce
residual organic matter.
· Tertiary treatment – to address different pollutants using
different treatment processes.
During
wastewater treatment, a mixture of solids and water is generated, known as
sludge.
According to
the IWA, the volume of sludge produced in a WWTP is only about 1% (dewatered
sludge is 0.5%) of the volume of influent wastewater to be treated.
To manage WWTPs
effectively and efficiently, it is absolutely necessary to extract waste sludge,
including inert solids and excess biomass, in order to prevent their
accumulation within the system.
In a WWTP the
types of sludge produced are:
· primary sludge – produced by settleable solids removed from raw
wastewater in primary settling; characterised by high putrescibility and good
dewaterability when compared to biological sludge; Total solids content in
primary sludge is in the range 2-5%.
· secondary sludge (also called biological sludge) – produced by
biological processes such as activated sludge or biofilm systems; contains
microorganisms grown on biodegradable matter (either soluble or particulate),
endogenous residue and inert solids not removed in the primary settling (where
a primary settler is present) or entering with the raw wastewater (where no
primary settler is present); TS content in secondary sludge is in the range
0.5-1.5%.
· chemical sludge – produced by precipitation of specific
substances (i.e. phosphorus) or suspended solids.
Wastewater pre treatment (preliminary treatment)
Pre-treatment
is necessary to remove anything that might interfere with subsequent treatment.
It can protect
raw water lifting systems and pipelines against blockages, as well as other
treatment equipment against abrasion and to generally remove anything that might
interfere with subsequent treatment.
They can also
help to reduce abrasion of mechanical parts and extend the life of the
sanitation infrastructure.
The following
constitute pre-treatment operations, although wastewater treatment plants can
comprise one of more of the options, depending on raw water quality: bar
screening; straining; comminution; grit removal; grease removal, frequently
combined with grit removal; oil removal; by-product treatment: grit and grease;
combined treatment of mains cleaning waste and of plant grit.
Advantages of
pre-treatment include relatively low capital costs and low to moderate
operating costs.
This is coupled
with the reduced risk of impairing subsequent conveyance and/or treatment
technologies. Meanwhile, the major disadvantage is frequent maintenance
required.
Often the first
unit operation used at wastewater treatment plants, screening is used to remove
objects to prevent damage and clogging of downstream equipment and piping.
Both coarse
screens and fine screens can be used in some modern wastewater treatment
plants.
Coarse screens
remove large solids, rags, and debris from wastewater, and typically have
openings of 6 mm (0.25 in) or larger.
Types of coarse
screens include mechanically and manually cleaned bar screens, including trash
racks.
Fine screens
are typically used to remove material that may create operation and maintenance
problems in downstream processes, particularly in systems that lack primary
treatment.
Typical opening
sizes for fine screens are 1.5 to 6 mm (0.06 to 0.25 in). Very fine screens
with openings of 0.2 to 1.5 mm (0.01 to 0.06 in) placed after coarse or fine
screens can reduce suspended solids to levels near those achieved by primary
clarification.
Three stages to wastewater treatment process
1) Primary
treatment
The objective
of primary treatment is the removal of settleable organic and inorganic solids
by sedimentation, and the removal of materials that will float (scum) by
skimming.
Approximately
25% to 35% of the incoming biochemical oxygen demand (BOD), 50 to 70% of the
total suspended solids (SS), and 65% of the oil and grease are removed during
primary treatment.
Some organic
nitrogen, organic phosphorus, and heavy metals associated with solids are also
removed during primary sedimentation but colloidal and dissolved constituents
are not affected.
2) Secondary
treatment
The objective
of secondary treatment is to remove the residual organics and suspended solids.
In most cases,
secondary treatment follows primary treatment and involves the removal of
biodegradable dissolved and colloidal organic matter using aerobic biological
treatment processes.
Aerobic
biological treatment is performed in the presence of oxygen by aerobic
microorganisms (principally bacteria) that metabolize the organic matter in the
wastewater, thereby producing more microorganisms and inorganic end-products.
Several aerobic
biological processes are used for secondary treatment.
These differ
primarily in the manner in which oxygen is supplied to the microorganisms and
in the rate at which organisms metabolize the organic matter.
Common
high-rate processes include the activated sludge processes, trickling filters
or biofilters, oxidation ditches, and rotating biological contactors.
In the
activated sludge process, the dispersed-growth reactor is an aeration tank or
basin containing a suspension of the wastewater and microorganisms, the mixed
liquor.
The contents of
the aeration tank are mixed vigorously by aeration devices which also supply
oxygen to the biological suspension.
Aeration
devices commonly used include submerged diffusers that release compressed air and
mechanical surface aerators that introduce air by agitating the liquid surface.
Hydraulic
retention time in the aeration tanks usually ranges from three to eight hours
but can be higher with high BOD wastewaters.
Following the
aeration step, the microorganisms are separated from the liquid by
sedimentation and the clarified liquid is secondary effluent.
A portion of
the biological sludge is recycled to the aeration basin as return activated
sludge (RAS) to maintain a high mixed-liquor suspended solids (MLSS) level.
The remainder
is removed as surplus activated sludge (SAS) or otherwise known as waste
activated sludge (WAS) from the process and sent to sludge processing to
maintain a relatively constant concentration of microorganisms in the system.
3) Tertiary
treatment
Tertiary and/or
advanced wastewater treatment is used to remove specific wastewater
constituents which cannot be removed by secondary treatment.
Nitrogen,
phosphorus, additional suspended solids, refractory organics, heavy metals and
dissolved solids can be removed using individual treatment processes.
However,
advanced treatment processes are sometimes combined with primary or secondary
treatment (e.g., chemical addition to primary clarifiers or aeration basins to
remove phosphorus) or used in place of secondary treatment (e.g., overland flow
treatment of primary effluent).
4 advanced waste water solutions
Multiple
advanced treatment solutions are available, driven by the need for improved
operational cost (OPEX), smaller plant footprints and more stringent
regulations governing discharge.
Below we have
highlighted four key waste water solutions.
1) Moving Bed Biofilm Reaction (MBBR) Technology
MBBR technology
employs thousands of polyethylene biofilm carriers operating in mixed motion
within an aerated wastewater treatment basin.
Each individual
biocarrier increases productivity through providing protected surface area to
support the growth of heterotrophic and autotrophic bacteria within its cells.
It is this
high-density population of bacteria that achieves high-rate biodegradation
within the system.
MBBR processes
can self-maintain an optimum level of productive biofilm which, when attached
to the mobile biocarriers within the system automatically responds to load
fluctuations, according to Headworks International.
2) Membrane
Bioreactors (MBR)
Membrane
bioreactor’ (MBR) is generally a term used to define wastewater treatment
processes where a perm-selective membrane, e.g. microfiltration or
ultrafiltration, is integrated with a biological process − specifically a
suspended growth bioreactor, according the MBR site.
MBRs differ
from ‘polishing’ processes where the membrane is employed as a discrete
tertiary treatment step with no return of the active biomass to the biological
process.
Almost all
commercial MBR processes available today use the membrane as a filter,
rejecting the solid materials which are developed by the biological process,
resulting in a clarified and disinfected product effluent.
3) Membrane
aerated biofilm reactor (MABR)
MABR systems
passively circulate oxygen through a spirally wound membrane at atmospheric
pressure.
MABR’s
self-respiring membrane allows bacteria to consume oxygen more readily for a
90% reduction in energy used for aeration.
The membrane
surface accumulates a biofilm of bacteria that establishes a simultaneous
nitrification-denitrification (SND) process to produce a high-quality,
low-nitrogen effluent suitable for reuse in irrigation.
4) Ultraviolet
(UV)
For advanced
wastewater treatment plants, ultraviolet (UV) technology has been included in
the tertiary treatment process.
This can allow
the wastewater treatment plant to meet even more stringent requirement, in some
cases for indirect and direct potable reuse and water reclamation.
The wavelengths
of UV light range between 200 and 300 nanometers (billionths of a meter).
Special
low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the
optimal wavelength for disinfection and ozone destruction.
Categorised as
germicidal, this means they are capable of inactivating microorganisms, such as
bacteria, viruses and protozoa.
The UV lamp
never contacts the water; it is either housed in a quartz glass sleeve inside
the water chamber or mounted external to the water which flows through UV
transparent Teflon tubes.
One notable
development to UV systems is the scaling up of light-emitting diode technology,
known as UV-LED, with 2018 witnessing a tipping point on power density and
purchasing price.
Waste water recovery
Water reuse is
a form of wastewater recovery whereby water can be extracted for purposes such
as agricultural and golf course irrigation, rather than being discharged to the
environment.
This ties in
with a growing trend to see wastewater treatment plants instead as resource
recovery centres.
Recovering the
water, energy, nutrients and other precious materials embedded in wastewater is
a key opportunity, according to the IWA.
Used water is
one of the most under-exploited resources available.
Water from
industrial or domestic use contains energy, water, organics, phosphates,
nitrogen, cellulose, rare earths, and other resources.
The IWA said
technologies are increasingly making resource recovery from wastewater
commercially feasible, including bio-gas, fertiliser, paper, metals, plastics
and, perhaps most importantly, it is a source of ‘new’ water.
Industrial Wastewater Treatment
Most water
reuse applications prior to the last decade were producing secondary quality
water for industrial or agricultural purposes.
These will
still provide major uses for lower grade reused wastewater. However, for
potable and some industrial purposes, a high level of treatment is required.
When addressing
the question of why reuse wastewater, one answer is because you've already paid
for the treatment so why not make the most of this resource.
Techniques for
potable water reuse can involve membrane-based techniques such as
ultrafiltration (UF) and reverse osmosis (RO), and using ultraviolet (UV) light
or ozone for disinfection.
Lately, these
are finding other applications in industry.
Other
techniques such as electrodialysis, ceramic membranes and advanced oxidation
are also being used in novel ways to enable wastewater reuse.
For potable
purposes, the industry has split wastewater reuse into indirect (IPR) and
direct (DPR) reuse, the latter requiring much more stringent standards and
approvals than the former.
The city-state
of Singapore has long been the pathfinder in reusing wastewater for potable
purposes - NEWater, as the government terms it.
Yet the small
cities of Big Spring and Wichita Falls in Texas and the much larger city of San
Diego in California will probably be better remembered as ushering in DPR
across the world.
The US already
hosts a world-leading example of IPR in the Orange County Groundwater
Replenishment System in California.
Meanwhile,
Australia boasts an equally large project, the 232,000 m3/day Western Corridor
Recycled Water Project in Queensland.
This has three
advanced wastewater treatment plants, which contribute reused water to industry
and agriculture.
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