What is Municipal Wastewater?
Municipal wastewater, commonly known as sewage,
consists of wastewater discharge from residential and restaurant bathrooms,
kitchen, dishwashers and washing machines. It includes bathroom and kitchen
wastewater from commercial and industrial facilities, as well. Municipal
wastewater also consists of treated or partially treated industrial wastewater that
is mixed with bathroom and kitchen wastewater.
If untreated municipal wastewater finds its way
to our agricultural lands, it will form septic odorous areas and pollute our
food and ground waters with bacteria and other harmful microorganisms for
years. Untreated wastewater that ends up in creeks, rivers, lakes and oceans promotes
the growth of algae and deprive marine life from oxygen and clean water to live
in. Humans and animals drinking or living near water contaminated with
municipal wastewater are susceptible to water and air borne diseases.
Governments around the world impose strict
regulations on the quality of wastewater discharge forcing municipalities,
industry and remote sites alike to treat wastewater before discharging it to
the environment.
What are the contaminants contained in
municipal wastewater?
Municipal wastewater contains tissue paper and
inorganic solids that people toss into toilets. It also contains organics
commonly represented as Biological Oxygen Demand (BOD). This BOD reflects the
amount of oxygen these organics will consume as they biologically decompose to
carbon dioxide and water molecules. Part of the BOD is present as soluble
organics and part of it is present as Total Suspended Solids (TSS).
While Fat, Oil and Grease (FOG) are essentially
large molecule organics that contribute to the BOD of wastewater, they are
always reported as a separate concentration because they are hard to treat. If
FOG is present in large concentrations, it’s typically removed before BOD and
TSS are removed. If present in low concentrations, then the wastewater
treatment plant is designed to consider the presence of residual FOG.
Municipal wastewater also contains Nitrogen and
Phosphorous commonly known as nutrients. If left untreated, nutrients will
promote the growth of algae and bacteria in water ways even if FOG, BOD and TSS
are removed. Most governments today require the removal of nutrients from
wastewater.
Conventional Municipal Wastewater Treatment
Plants
Traditional municipal wastewater treatment
plants consist of mechanical screening to remove large items followed by
aeration tank(s) and gravity clarifier(s). Air blowers deliver atmospheric air
to diffusers at the bottom of the aeration tank to supply the bacteria in the
wastewater with enough oxygen to grow and digest the organics to carbon dioxide
and water. The solids in the aeration tanks are called Mother Liquor Suspended
Solids (MLSS). The MLSS is a mixture of the bodies of dead and alive bacteria
that multiply by consuming the soluble and suspended organics in the
wastewater. These solids settle at the bottom of the gravity clarifiers where
most of them are recirculated back to the aeration tanks to maintain a typical
MLSS concentration of 3,000 mg/L.
The treated water effluent from the clarifier(s)
is often chlorinated for disinfection before discharge to waterways.
Alternatively, the treated water passes through Ultraviolet Light (UV) machines
for disinfection.
A typical raw municipal wastewater contains
250-300 mg/L BOD and 250-300 mg/L TSS. The MLSS in a typical aeration/clarification
plant is about 3,000 mg/L. The effluent of these plants contains 20 mg/L BOD
and 10-20 mg/L TSS. This wastewater is still acceptable by many municipalities
for discharge to water ways or irrigation of non-crop producing plants or lawns.
Conventional wastewater treatment plants occupy
a large footprint, because the typical hydraulic retention times (HRT) inside
these systems is 12-24 hours. These plants are only capable of partial Nitrogen
and Phosphorus reduction. If near complete nitrogen removal is required then
Anoxic tanks are added ahead of the aeration tank. Anoxic tanks receive a
mixture of raw wastewater and aerated water from the aeration tanks. The Anoxic
tanks are not aerated themselves. These conditions promote the growth of different
types of bacteria that convert organic nitrogen to Nitrates.
Conventional wastewater treatment plants are
also susceptible to sudden changes in raw wastewater quality and temperature.
Moving Bed Bio-Reactor (MBBR) Technology
The MBBR technology utilizes floating plastic
beads placed in the aeration tank. As bacteria grows inside the aeration tanks
they start to form colonies that attach to the beads, growing the concentration
of MLSS inside the aeration tank to about 15,000 mg/L. This higher
concentration of available microorganisms accelerates the biological
decomposition of organics in the wastewater. Furthermore, the anaerobic
conditions inside the beads cause nitrification/ denitrification reactions that
reduce the organic nitrogen in the wastewater to nitrogen. MBBR systems require
short hydraulic retention times. Municipal MBBR systems require just 6 hours of
hydraulic. Therefore, they occupy about one half to one quarter the footprint
of conventional wastewater treatment plants.
MBBR systems produce fine TSS that is hard to
settle in the clarifiers. While producing BOD of less than 10 mg/L, the TSS if
the effluent of MBBR systems can be in the range of 20-50 mg/L. Unless
acceptable for remote site, this TSS must be further reduced with expensive
Dissolved Air Floatation systems installed after the gravity clarifiers or
expensive inclined plate clarifiers that replace conventional gravity
clarifiers.
Membrane Bio-Reactors (MBR) Technology
The MBR technology utilizes microfiltration of
ultrafiltration membranes placed at the end of the aeration tanks. With
recirculation, these membranes increase the MLSS inside the aeration tank to
about 15,000 mg/L. The pore size of the membranes is less than 1-micron.
Therefore, the high quality treated wastewater has BOD less than 10 mg/L and
TSS less than 5 mg/L.
MBR systems provide complete nitrification and
semi de-nitrification. Membrane bioreactor (MBR) deals with a specialized
wastewater treatment technology designed for different municipal and industrial
applications with plant sizes that approach 48 million liters per day. Membrane
bioreactor machines are introduced in applications such as water reuse, turnkey
projects, new housing developments, retrofits, parks, and hotels. It comes
across between ultrafiltration or microfiltration with a suspended growth
bioreactor.
When there is a lot of demand for recycling
wastewater and stricter environmental regulations,
membrane bioreactor machines
would be rational solutions for existing and future wastewater treatments. They
have specialized features such as simplicity of replacing elements, lowered
module height, and fine-bubble air diffusers. Each element within the MBR can
be easily taken out when it needs to be inspected or replaced. The module
height of a membrane bioreactor machine is lowered to suit containerized
package systems and several applications with height limitations.
Fine-bubble air diffusers help MBRs by
massively decreasing the daily cleaning operation of air diffusers and raising
the efficiency of oxygen dissolution. MBR technologies can generate effluent of
high quality enough to be dumped to coastal, surface or brackish waterways or
to be recollected for urban irrigation when being used with domestic
wastewater. MBR has other advantages
such as small footprint, simple retrofit and upgrade of old wastewater
treatment plants. MBR technologies can function at higher mixed liquor
suspended solids (MLSS) concentrations similar to conventional settlement
separation systems, therefore, decreasing the reactor volume and achieving the
same loading rate.
Pure Aqua’s Containerized BioSmart BMBR
Treatment Plants
Pure Aqua’s series of containerized BioSmart
BMBR are compact
wastewater treatment plants
pre-designed specifically to treat
municipal or domestic wastewater discharge from remote camp sites, resorts and
small municipalities. These pre-engineered systems come with fine 1-2 micron
screens, fine bubble diffusers and state-of-the-art blowers and 0.4 micron
membranes producing high quality treated water. They also come with
automatically controlled membrane cleaning sequences that require no
supervision.
A BioSmart BMBR system is a plug and play
system with minimum required inlet/ outlet plumbing and electrical wiring.
The BOD and TSS of treated municipal wastewater
produced by Pure Aqua’s BMBR systems are 10 mg/L and 5 mg/L respectively. This
water can be safely used for irrigation or reuse for non-potable uses.
For more information about Wastewater Treatment Systems, please go to: https://www.pureaqua.com/waste-water-treatment/
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