Sewage/ Wastewater Treatment Technologies
The
most common suspended growth process used for municipal wastewater treatment is
the activated sludge process. The municipal
wastewater treatment is the BOD-removal. The removal of BOD is done by a
biological process, such as the suspended
growth treatment process. This biological process is an aerobic process
and takes place in the aeration tank, in where the wastewater is aerated with
oxygen. By creating good conditions, bacteria will grow fast. The grow of
bacteria creates flocks and gases. These flocks will removed by a secondary
clarifier. In the activated sludge
process, the dispersed-growth reactor is an aeration tank or basin containing
as 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 3 to 8 hours but can
be higher with high BOD5 wastewaters. Following the aeration step,
the microorganisms are separated from the liquid by sedimentation and the
clarified liquid is secondary effluents. A portion of the biological sludge is
recycled to the aeration basin to maintain a high mixed-liquor suspended solids
(MLSS) level. The remainder is removed from the process and sent to sludge
processing to maintain a relatively constant concentration of microorganisms in
the system. Several variation of the basic activated sludge process, such as
extended aeration and oxidation ditches, are in common use, but the principal
are similar:
2. Trickling Filters
A
trickling filter or biofilter consists of a basin or tower filled with support
media such as stones, plastic shapes, or wooden slats. Wastewater is applied
intermittently, or sometimes continuously, over the media. Microorganisms
become attached to the media and form a biological layer or fixed film. Organic
matter in the wastewater diffuses into the film, where it is metabolized.
Oxygen is normally supplied to the film by the natural flow of air either up or
down through the media, depending on the relative temperatures of the
wastewater and ambient air. Forced air can also be supplied by blowers but this
is rarely necessary. The thickness of the biofilm increases as new organisms
grow. Periodically, portions of the film 'slough off the media. The sloughed
material is separated from the liquid in a secondary clarifier and discharged
to sludge processing. Clarified liquid from the secondary clarifier is the
secondary effluent and a portion is often recycled to the biofilter to improve
hydraulic distribution of the wastewater over the filter.
3. Rotating Biological Contactors
Rotating
biological contactors (RBCs) are fixed-film reactors similar to biofilters in
that organisms are attached to support media. In the case of the RBC, the
support media are slowly rotating discs that are partially submerged in flowing
wastewater in the reactor. Oxygen is supplied to the attached biofilm from the
air when the film is out of the water and from the liquid when submerged, since
oxygen is transferred to the wastewater by surface turbulence created by the
discs’ rotation. Sloughed pieces of biofilm are removed in the same manner
described for biofilters.
High-rate biological treatment processes, in
combination with primary sedimentation, typically remove 85 % of the BOD5
and SS originally present in the raw wastewater and some of the heavy metals.
Activated sludge generally produces an effluent of slightly higher quality, in
terms of these constituents, than biofilters or RBCs. When coupled with a
disinfection step, these processes can provide substantial but not complete
removal of bacteria and virus. However, they remove very little phosphorus,
nitrogen, non-biodegradable organics, or dissolved minerals.
UASB is an
anaerobic process whilst forming a blanket of granular sludge and suspended in
the tank. Wastewater flows upwards through the blanket and is processed by the
anaerobic microorganisms. The upward flow combined with the settling action of
gravity suspends the blanket with the aid of flocculants. The blanket begins to
reach maturity at around 3 months. Small sludge granules begin to form whose
surface area is covered in aggregations of bacteria. In the absence of any support
matrix, the flow conditions create a selective environment in which only those
microorganisms, capable of attaching to each other, survive and proliferate.
Eventually the aggregates form into dense compact biofilms referred to as
"granules".
Fine granular sludge
blanket acts as a filter to prevent the solids in the incoming wastes to flow
through as the liquid part does. So if the hydraulic retention time (HRT) does
not change, which is limited to 1-3 days (the bigger the digester, the shorter
time it is, because the size costs money), the solid retention time (SRT) can
be 10-30 days or more for more effective digestion, depending on the shape of
the digestion chamber. It means that the digester becomes much more efficient
without having to increase the size, which costs money. Wageningen University
in the Netherlands has started to do R & D along these lines.
Standing and hanging
baffles are used, with a conic separation with a small outlet at the center
will be much more effective to keep the anaerobic sludge blanket in the lower
part of the digester. This will act as a very good filter to retard the flow of
solids in the wastes and prolong the solid retention time for more bacterial
action. However, the digester will be more economic if the loading can be
increased for a specific size of digester with the conic separation. COD
reduction of 58% now obtained is adequate, and no attempt should be made to
increase the bacterial action at such high costs. It is better to use much cheaper
open tanks and basins for more effectiveness and efficiency, as in the
IF&WMS.
1. Hydrolysis or
solubilization - The first phase takes 10-15 days, and until the
complex organics are solubilized, they cannot be absorbed into the cells of the
bacteria where they are degraded by the endoenzymes;
2. Acidogenesis or
acetogenesis - The result from stage one utilized by a second
group of organisms to form organic acids;
3. Methanogenesis - The methane-producing (methanogenic) anaerobic bacteria then use the
product of (2) to complete the decomposition process.
Introduction
Waste water stabilization
pond technology is one of the most important natural methods for wastewater
treatment. Waste stabilization ponds are mainly shallow man-made basins
comprising a single or several series of anaerobic, facultative or maturation
ponds The primary treatment takes place in the anaerobic pond, which is mainly
designed for removing suspended solids, and some of the soluble element of
organic matter (BOD). During the secondary stage in the facultative pond most
of the remaining BOD is removed through the coordinated activity of algae and
heterotrophic bacteria. The main function of the tertiary treatment in the
maturation pond is the removal of pathogens and nutrients (especially
nitrogen). Waste stabilization pond technology is the most cost-effective wastewater
treatment technology for the removal of pathogenic micro-organisms. The
treatment is achieved through natural disinfection mechanisms. It is
particularly well suited for tropical and subtropical countries because the
intensity of the sunlight and temperature are key factors for the efficiency of
the removal processes.
Water treatment in waste
stabilization ponds
(a)
Anaerobic ponds
These units are the
smallest of the series. Commonly they are 2-5 m deep and receive high organic
loads equivalent to100 g BOD/m3 d. These high organic loads produce strict
anaerobic conditions (no dissolved oxygen) throughout the pond. In general
terms, anaerobic ponds function much like open septic tanks and work extremely
well in warm climates. A properly designed anaerobic pond can achieve around
60% BOD removal at 20° C. One-day hydraulic retention time is sufficient for
wastewater with a BOD of up to 300 mg/l and temperatures higher than 20° C.
Designers have always been preoccupied by the possible odour they might cause.
However, odour problems can be minimised in well designed ponds, if the SO42-
concentration in wastewater is less than 500 mg/l. The removal of organic
matter in anaerobic ponds follows the same mechanisms that take place in any
anaerobic reactor.
(b)
Facultative ponds
These ponds are of two
types: primary facultative ponds receive raw wastewater, and secondary
facultative ponds receive the settled wastewater from the first stage (usually
the effluent from anaerobic ponds). Facultative ponds are designed for BOD
removal on the basis of a low organic surface load to permit the development of
an active algal population. This way, algae generate the oxygen needed to
remove soluble BOD. Healthy algae populations give water a dark green colour
but occasionally they can turn red or pink due to the presence of purple
sulphide-oxidising photosynthetic activity. This ecological change occurs due
to a slight overload. Thus, the change of colouring in facultative ponds is a
qualitative indicator of an optimally performing removal process. The
concentration of algae in an optimally performing facultative pond depends on
organic load and temperature, but is usually in the range 500 to 2000 μg
chlorophyll per litre. The photosynthetic activity of the algae results in a diurnal
variation in the concentration of dissolved oxygen and pH values. Variables
such as wind velocity have an important effect on the behaviour of facultative
ponds, as they generate the mixing of the pond liquid. As Mara et al. indicate, a good degree of mixing ensures a
uniform distribution of BOD, dissolved oxygen, bacteria and algae, and hence
better wastewater stabilization. More technical details on the efficiency of
the process and removal mechanisms can be found in Mara et al. and Curtis.
(c)
Maturation ponds
These ponds receive the
effluent from a facultative pond and its size and number depend on the required
bacteriological quality of the final effluent. Maturation ponds are shallow
(1.0-1.5 m) and show less vertical stratification, and their entire volume is
well oxygenated throughout the day. Their algal population is much more diverse
than that of facultative ponds. Thus, the algal diversity increases from pond
to pond along the series. The main removal mechanisms especially of pathogens and
faecal coliforms are ruled by algal activity in synergy with photo-oxidation.
On the other hand,
maturation ponds only achieve a small removal of BOD, but their contribution to
nitrogen and phosphorus removal is more significant. A report on total nitrogen
removal of 80% in all waste stabilization pond systems, which in this figure
corresponds to 95% ammonia removal. It should be emphasised that most ammonia
and nitrogen is removed in maturation ponds. However, the total phosphorus
removal in WSP systems is low, usually less than 50%.
Operation
and maintenance
Starting up the system,
Once the construction of the system has been completed it should be checked
that all ponds are free of vegetation. This is very important if the waste
stabilization pond is not waterproof. Facultative ponds should be filled prior
to anaerobic ponds to avoid odour release when anaerobic pond effluent
discharges into an empty facultative pond . Anaerobic ponds should be filled
with raw wastewater and seeded whenever possible with biosolids from another
anaerobic reactor. Later, the anaerobic ponds can be gradually loaded up to the
design’s loading rate. This gradual loading period can be from one to four
weeks depending on the quality of the digester used or in case the pond was not
seeded during the start-up procedure. It is important to measure the pH in the
anaerobic pond and maintain it above 7 to permit the development of the
methanogenic bacterial population. During the first month it may be necessary
to add lime, to avoid the acidification of the reactor.
Initially, facultative and maturation ponds should be filled with
freshwater from a river, lake or well, so as to permit the gradual development
of the algal and heterotrophic bacterial population. If freshwater is unavailable,
facultative ponds should be filled with raw wastewater and left for three to
four weeks to allow the aforementioned microbial populations to develop. A
small amount of odour release is inevitable during the implementation of the
latter method in the facultative pond.
Routine
maintenance
Once the waste
stabilization ponds have started to operate, it is necessary to carry out
regular routine maintenance tasks. Although simple, these tasks are essential
to the good operation of the system.
Ø Removal of screening and grit retained in the inlet works during the
preliminary treatment.
Ø Cutting, pruning and removing the grass and vegetation that grows on the
embankment to prevent it from falling into the pond and generating the
formation of mosquito breeding habitats. The use of slow-growing grass or
vegetation is recommended to minimise the frequency of this task.
Ø Removal of floating scum and macrophytes (e.g. Lemna spp.) from
facultative and maturation ponds to maximise photosynthesis and surface re-aeration,
and prevent fly and mosquito breeding.
Ø Spraying the scum on the surface of anaerobic ponds (which should not be
removed as it aids the treatment process). In the event fly breeding is
detected this material should be sprayed with clean water.
Ø Removal of any accumulated solids in the pond’s inlets and outlets.
Ø Repair of any damage to the embankments caused by rodents or other
animals.
Ø Repair of any damage to external fences and gates or points of access to
the system.
The operator responsible should register these activities in a pond
maintenance record sheet. Usually this operator is also in charge of taking
samples and measurements of the pond’s effluent flow.
The mechanic-biological purification of the waste
water takes place in one or more aerated lagoons according to the size of the
plant, which are followed by an non-aerated sedimentation and polishing
pond. The sewage coming from the
canalisation is normally led directly into the first aerated lagoon without
mechanical pre-purification. So the continuous disposal of screenings, sand and
sedimentation sludge and its maintenance efforts can be omitted. Coarse stuff, sand and heavy sludge settle in
the inlet zone while dissolved contaminant are distributed in the whole first
lagoon. Liable to putrefy matter should
mainly be stabilized by aerobic processes to avoid odours and digested sludge
coming up to the water surface.
According to our experience sludge at the inlet zone of the first
aerated waste water lagoon has to be removed at regular intervals of several
years. To exhaust and bring the sludge out liquid manure-vacuum-tankers are
used. Floating solids are retained by a
scum board in the inlet area. They should be removed once or twice a week with
a rake.
To design bigger plants (> 1,500 – 2,000 p.e) it has
of course to be considered carefully if a mechanical pre-purification of the
waste water by a fine screen or a sieve offer still more advantages. Purification processes in an aerated waste
water lagoon are best compared with those of a loaded water flow. Unlike activated sludge plants where
suspended activated sludge eliminates the dissolved contaminant out of the
waste water the active biomass is essentially as a fixed biological film at the
bottom of the lagoon. Basic requirement
for an extensive biological reduction of the dissolved contaminant is therefore
in addition to a sufficient oxygen transfer the effective circulation and
mixing of the lagoons. So stagnant zones can be avoided and an everlasting
exchange of water in the area of the fixed biological film at the bottom of the
lagoon is ensured.
Treatment of storm-water
Due to the long retention times (approx. 10 days) and the
low load aerated waste water lagoons dispose of a high buffering capacity
compared to the waste water load.
Unlike activated sludge plants there is no danger that the active biomass is
carried out at hydraulic overload. Accordingly
a simultaneous rainwater treatment is easily possible. There are often no
special measures and the whole rainwater is led through the lagoons.
It is a good solution to operate the first aerated lagoon
in backwater. When reaching the maximum
water level a pond overflow structure will go into operation. In the case the
first aerated lagoon is designed as a storm-water tank to retain the first
amount of discharge storm-water, the storm-water overflow in front of the pond
overflow will go into operation.
Aeration of waste water lagoons
FUCHS Spiral Aerators meet the requirements of an
aeration system for the aeration, mixing and circulation in a special wax. For
information concerning design and operation of these machines please see our
prospectus « FUCHS Spiral Aerator ». If the Spiral Aerators have a suitable
position in hydraulic well designed ponds an even circulation flow is formed at
the lowest power requirement. It includes the whole volume of the pond and
grants an even oxygen transfer and mixing. In bigger, round or square shaped ponds also
the FUCHS Circulation Aerator is in operation. According to their flow pattern
they are installed in the middle of the ponds. They mainly cause a vertical
shifting and mixing of the waste water and complete the flow developed by the
Spiral Aerators. Both aerator types have
a robust construction and are almost maintenance-free. Due to their little
weight a fast and easy installation without hoist is possible. It also causes
no difficulties and no expenditure to dismount the machines for example for
deslurrying the pond. The machines are
preferably installed on a floating devices. If a constant water level is
granted, the aerators can also be installed on bridges.
The machines have the following advantages : high
circulation and mixing capacity minimum maintenance requirements no danger of
clogging, also at intermittent operation or power failure no spray water no
odour problems no noise problems no frost problems no big fan station and
compressed air pipes
Oxidation Ponds are also known as stabilization ponds or lagoons. They are used for simple secondary treatment of sewage effluents. Within an oxidation pond heterotrophic bacteria degrade organic matter in the sewage which results in production of cellular material and minerals. The production of these supports the growth of algae in the oxidation pond. Growth of algal populations allows furthur decomposition of the organic matter by producing oxygen. The production of this oxygen replenishes the oxygen used by the heterotrophic bacteria. Typically oxidation ponds need to be less than 10 feet deep in order to support the algal growth. In addition, the use of oxidation ponds is largely restricted to warmer climate regions because they are strongly influenced by seasonal temperature changes. Oxidation ponds also tend to fill, due to the settling of the bacterial and algal cells formed during the decomposition of the sewage. Overall, oxidation ponds tend to be inefficient and require large holding capacities and long retention times. The degradation is relatively slow and the effluents containing the oxidized products need to be periodically removed from the ponds.
8. Karnal Technology
The Karnal Technology involves
growing tree on ridges 1m wide and 50cm high
wand disposing of the untreated sewage in furrows. The amount of
the sewage/ effluents to be disposed off depends upon the age, type of plants,
climatic conditions, soil texture and quality of effluents. The total
discharge of effluent is so regulated that it is consumed within 12-18 hours
and there is no standing water left in the trenches. Through this
technique, it is possible to dispose off 0.3 to 1.0 ML of effluent per day per
hectare. This technique utilizes the entire biomass as living filter for
supplying nutrients to soil and plant; irrigation renovates the effluent for
atmospheric re-charge and ground storage. Further, as forest plants are
to be used for fuel wood, timber or pulp, there is no chance of pathogens,
heavy metals and organic compounds to enter into the human food chain system, a
point that is a limiting factor when vegetables or other crops are grown with
sewage.
Though most of the plants are
suitable for utilizing the effluents, yet, those tree species which are fast
growing can transpire high amounts of water and are able to with stand high
moisture content in the root environment are most suitable for such
purposes. Eucalyptus is one such species, which has the capacity to
transpire large amounts of water, and remains active through out the year.
Other species suitable for this
purpose are poplar and leucaena. Out of these three species, eucalyptus
seems to be the best choice as poplar remains dormant in winter and thus cannot
bio-drain effluent during winter months. However, if area is available
and the volume of effluent is small, a combination of popular and eucalyptus is
the best propagation. This technology for sewage water use is relatively cheap
and no major capital is involved. The expenditure of adopting this
technology involves cost of making ridges, cost of plantation and their
care.
This system generates gross returns
from the sale of fuel wood. The sludge accumulating in the furrows along
with the decaying forest litter can be exploited as an additional source of
revenue. As the sewage water itself
provides nutrients and irrigation ameliorates the sodic soil by lowering the
pH, relatively unfertile wastelands can be used for this purpose. This
technology is economically viable as it involves only the cost of water
conveyance from source to fields for irrigation and does not require highly
skilled personnel as well. This technology seems to be most appropriate
and economical viable proposition for the rural areas as this technology is
used to raise forestry, which would aid in re-storing environment and to generate
biomass.
9. Duckweed
Duckweeds are very
common in Iowa waters. These aquatic plants are the world smallest and simplest
flowering plants. Duckweeds are floating plants that grow on the surface of
still or slow moving waters during warmer weather. Because duckweeds usually
reproduce by budding, they can multiply very quickly and cover the entire
surface of a pond in a short amount of time. Small numbers of duckweeds will
not harm a pond, but large numbers will block sunlight from entering the pond and
upset the ponds oxygen balance, placing the fish population in danger. The Lemna
spp. are the most common duckweeds. Lemna grow up to 4 mm (5/32 in)
wide and have a single root dangling from the ÒleafÓ of the plant. Duckweeds do
not have true leaves or stems; the roundish, flattened Òleaflike Ó part of the
plant is called a frond.
Another type, watermeal
(Wolffia spp.), are the smallest of the duckweeds. These plants are so
tiny that they look like grains of green meal floating on the water surface. They are generally less than 1 mm
(1/32 in) wide and barely visible as individuals. This type of duckweed does not
have roots.
Control
Many
times control is necessary because the duckweeds reproduce rapidly and can
cover a pond causing oxygen problems.
Biological Control
Biological control refers to the use of one organism to control the
growth of another. Biological control of duckweeds may be accomplished through
the use of grass carp, koi, or goldfish. These fish will all eat duckweed, but
results are highly variable. Biological control is much more effective if
implemented before the duckweed become a problem; once established, biological
controls are not effective since duckweed reproduce so quickly.
Lemna spp. of duckweed are tiny plants that can quickly spread over a pond’s
surface.
Wolffia spp. of duckweed are so tiny, they look like green grains of meal
sprinkled on the pond surface.
Chemical Control
Chemical control (using
herbicides) is probably the most effective way to control the duckweeds. Diquat
and 2,4-D (liquid ester formulation) are sold under various trade names and
both have good control of duckweed. Sonarä (fluridone) has excellent control of
duckweed. Only fluridone applications allow for fair to good control of watermeal.
Both 2,4-D and diquat have varying water use restrictions depending on
formulation and rate. Fluridone does not have restrictions on drinking (by
humans or livestock), swimming, or fish consumption after application. However,
a restriction of 30 days is required before irrigation with treated water. For
good control of Duckweed, 2,4-D must be
used as a liquid ester formulation; however,
the liquid ester formulation is toxic to fish. Therefore, 2,4-D formulations should
be used with extreme caution when treating ponds with fish or only used for ornamental
ponds without fish. 2,4-D is a translocated herbicide and kills plants over
time. Treatment with 2,4-D formulations cost approximately
$50-100/surface acre.
One trade name of
diquat, Rewardä, is applied at a rate of 1 gallon/surface acre of water. At
this rate, approximate cost of treatment is $150-250/surface acre. However,
diquat is a contact herbicide and may be used as a foliar application, which
could reduce the cost of treatment substantially. When using diquat as a foliar
application, an approved nonionic surfactant is required. Also, diquat is
tightly bound to clay and is not effective in muddy water. Diquat kills plants
quickly, so only small areas at a time should be treated when dense vegetation
is present. Small treatments help to avoid pond oxygen depletion when large
amounts of vegetation are killed. Sonarä (fluridone) is a translocated
herbicide that kills plants over a long period of time (30-90 days). Fluridone
is not effective as a spot treatment; the entire pond must be treated to
control duckweeds. In water, Sonarä is applied at the rate of 0.16 Ð 0.40 quarts/surface
acre. The cost of treatment is approximately $100-250/surface acre. The rates
and prices given are only approximations and will vary depending on the manufacturer,
supplier, and extent of vegetation coverage. As always, read and follow
label directions of the particular herbicide being used.
The user is
always responsible for the effects of herbicide residues on livestock and crops,
as well as problems that could arise from drift or movement of the herbicide
from his or her property to that of others. Prepared by Joe Morris, extension
aquaculture specialist and Charles Mischke, Department of Animal Ecology, Iowa
State University.
The information given
herein is for educational purposes only. Reference to commercial products or
trade names is made with the understanding that no discrimination is intended and
no endorsement by the Cooperative Extension Service is implied.
10. Fluidized Bed Reactor
Aerobic fluidized bed reactors (FBRs) are used as a new
technology in wastewater treatment. An aerobic fluidized bed reactor with
granulated activated carbon (GAC) as carrier material can be operated under
different conditions, including batch-loading, semi continuous loading, and
continuous loading.
The basic idea behind the Fluidized
Bed Reactor is to have a continuous operating non-clogging bio film reactor
which requires (1) no back-washing, (2) has low head loss and (3) high specific
bio film surface area. This is achieved by having the biomass to grow on small
carrier elements that move with the liquid in the reactor. The movement within
the reactor is generated by aeration in the aerobic reactor. These bio-film
carriers are made of special grade plastic density close to that of water.
The fluidized bed reactor employs
fixed film principle and makes the treatment process more user friendly because
it does not require sludge recycle i.e. synonymous with conventional ASP. The
absence of sludge recycle frees the operator from the enormous task of
measurement and monitoring MLSS levels in the tank and adjusting recycle
rations continuously, due to fluctuating inlet COD loads. FBR produces small
quantity of sludge which requires no further treatment.
Fluidized Bed Reactor technology is
used in small Sewage Treatment Plants for treating city wastewater, industrial
sewage treatment plant from food waste, paper waste and chemical waste
etc. Due to fixed film nature, these
plants accept shock loads much better than those employed for suspended growth
process. Fluidized Bed Reactors are generally tall (6 m and above), thereby
reducing cross-sectional area further.
11. Sequential Batch Reactor
In this process, the raw sewage free
from debris and grit shall be taken up for biological treatment for removal of
organic, nitrogen and phosphorus. The activated sludge bio-system is designed
using Advanced Cyclic Activated Sludge Technology which operates on extended
aeration activated sludge principle for the reduction of carbonaceous BOD,
Nitrification, Denitrification as well as phosphorus removal using energy
efficient fine bubble diffused aeration system with automatic control of air
supply based on oxygen uptake rate.
In this form, the sequences of fill,
aeration, settle and decant are consecutively and continuously operated all in
the same tank. No secondary clarifier system is required to concentrate the
sludge in the reactor. The return sludge is recycled and the surplus is wasted
from the basin itself. The complete biological operation is divided into the
following cycles:
These phases in a sequence
constitute a cycle. During the period of a cycle, the liquid volume inside the
Reactor increases from a set operating bottom water level. During the
Fill-Aeration sequence mixed liquor from the aeration zone is recycled into the
Selector. Aeration ends at a predetermined period of the cycle to allow the
biomass to flocculate and settle under quiescent conditions. After a specific
setting period, the treated supernatant is decanted, using a moving weir
Decanter. The liquid level in the Reactor is so returned to bottom water level
after which the cycle is repeated. Solids are separated from the reactor during
the decanting phase. The system selected is capable of achieving the following:
i)
Bio-degradation
of organics present in the wastewater by Extended Aeration Process.
ii) Oxidation of sulphides in the wastewater
iii) Co-current nitrification and denitrification of Ammonical
nitrogen in the aeration zone.
iv) Removal of
phosphorous
Tertiary and/or advanced wastewater treatment is employed when specific wastewater constituents which cannot be removed by secondary treatment must be removed. The treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. Because advanced treatment usually follows high-rate secondary treatment, it is sometimes referred to as tertiary treatment. 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).
Chrome Recovery System
This system based on ion exchange
principle, will remove both Hexavalent chromium as anion and Trivalent
chromium, Nickel, Iron etc. as cation, in a two bed ion exchange system.
The static rinse after chrome
palting drag out tank, containing Hexavalent chrome ions and othe cationic
impurities like trivalent chrome, Nickel, iron, copper etc. is pumped through
chrome recovery coloumns. The strongly acidic cation exchanger removes trivalent
chromium (and other cations) while the macroracticular weak base anion exchange
resin removes the haxavalent chrome. This produces deionised water which can be
recycled in a closed loop system.
The hexavlent chromium is eluted
from anion exchanger with sodium Hydroxide in a concentrated solution. This
solution is further passed through a third column containing strongly acidic
resin in the hydrogen form. to recover chromic acid which can be recycled to
the plating bath after concentrating in an evaporator.
Chrome Recovery and Recycling
A proportion of pollution generated
from leather manufacturing can be contributed to the inefficiency of chemical
use in leather processing and to organic substances derived from the hides
during processing. In particular, the overall tanning processes
performed in drums can be characterised by a high
consumption of water and tanning agents, most of which are found in the final
wastewater. To increase the efficiency of leather production, chromium is added
in excess and is only partly taken up by the leather. Significant chromium
savings can be achieved by applying modern chrome recovery and recycling
technologies, thus reducing environmental impacts.
The novel chrome recycling process
uses robust and easy maintainable microfiltration membranes. The porous
Polyester cloth retains and concentrates alkaline chrome hydroxide, achieving
chrome concentrations of up to 30 g/l Cr3+ and a volume reduction of up to 90
%. The highly concentrated chrome hydroxide can be then acidified with
sulphuric acid and polished with the same membrane plant. Fats, proteins and
fibres are retained and high quality chrome liquors are ready for re-use for
tanning.