Wastewater Treatment

Contents

1. Wastewater Treatment: Overview
2. Treatment Process

2.1 Coarse screen
2.2 Coarse screens: Trouble shooting
2.3 Primary Sedimentation tanks: Trouble shooting
2.4 Grit Removal
2.5 Bio-Filters
2.6 Bio-Filters: Trouble shooting
2.7 Activated sludge: Overview
2.8 Sludge digestion: Overview
2.9 2Sludge thickening: Trouble shooting for gravity
thickening
2.10 Effluent disinfection: Overview of methods



Wastewater Treatment

Overview – Basic principles of treatment

The treatment of sewage involves the systematic removal or conversion of the harmful
constituents present in the sewage. A typical and logical sequence of waste water treatment
processes are as follows:

Preliminary treatment

Materials such as rags, plastics, sand, metal particles, etc. are transported through the sewers
with the wastewater. Although only a small proportion of the total wastewater flow, it can
have adverse effects on further treatment processes and can cause damage to plant
equipment and must be removed. Preliminary treatment of wastewater occurs at the head of
the works and generally includes screening, grit removal and flow measurement.

Primary treatment

Organic and inorganic solids are removed by sedimentation, and floatable material are
removed by skimming. Typically. a BOD reduction of 25% to 50% is achieved and 50% to 70%
of the SS (suspended solids), and 65% of the oil and grease are removed. “A primary
sedimentation tank is defined as a tank in which wastewater is retained long enough to bring
about sedimentation of suspended matter but short enough to prevent anaerobic
decomposition of the sludge. “

Secondary treatment

“Secondary treatment is the second step in most waste treatment systems during which bacteria
consume the organic parts of the wastes. This is accomplished by bringing the sewage, bacteria
and oxygen together in trickling filters or within an activated sludge process.”. Micro- organisms
and oxygen are utilized during secondary treatment to stabilize the sewage after primary
treatment and 85% to 95% of the SS and the BOD load can typically be removed. Secondary
treatment processes include percolating filters (trickling filters or rotating biological filters),
rotating biological contractors, and activated sludge processes, which normally consist of
aeration tanks (where air (oxygen) is injected into the primary treated wastewater),
sedimentation tanks (where the activated sludge is separated from the liquid) and final
clarifiers.

Wastewater Treatment
Overview – Basic principles of treatment

Tertiary treatment

“Tertiary treatment is the advanced treatment process, following secondary treatment of waste
water, that produces high—quality water. Tertiary treatment includes removal of nutrients such as
phosphorus and nitrogen and practically all suspended and organic matter from waste water.”
During tertiary treatment (which includes filtration, phosphorus removal, ammonia stripping
and other special treatments), specific constituents are removed. Further treatment may
include sand filtration, wetlands or other advanced treatment processes.

Disinfection

Methods for disinfection are:
Chemical, e.g. chlorination and ozonation
Physical, e.g. ultraviolet radiation and microfiltration
Biological, e.g. detention ponds

References: Ref.1. Water Institute of Southern Africa (2002): Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.
Ref.2. Organisation for Economic Co-operation and Development (OECD): Glossary of Statistical Terms. http://stats.oecd.org/glossary/index.htm

Wastewater Treatment
Coarse Screens

Sewage passes through bar screens for removal of larger objects (rags, plastics, tins, wood,
etc.). Automatic or manual bar screen cleaners remove the larger objects from the raw sewage.
The collected material is temporarily stored to be transferred later to a landfill site. The action
of the bar screen equipment is paced according to the amount of incoming solids and the flow
rate.
The amount and type of solids trapped depends on the size of the gaps in the screen. Typical
opening sizes (space between bars) for a coarse screen are between 10 and 25 mm.
Coarse screens have little or no effect on organic and suspended solids loads of the waste
water. The primary function is to protect the downstream equipment of the waste water
treatment plant against physical damage.
The amount of material removed will depend on the characteristics (activities, reticulation
method, etc.) of the area serviced. Future planning will depend on accurate past recordings of
flow and volume of screenings removed (For further reading refer to WISA, 2002).

Ref: Water Institute of Southern Africa (2002):Handbook for the operation of waste
water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment
Coarse Screens: Troubleshooting

Wastewater Treatment
Primary Sedimentation Tanks: Troubleshooting

Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants.
ISBN 0-958-45346-2.

Wastewater Treatment

Grit Removal

Purpose of grit removal

Wastewater grit materials (known as detritus) include sand, silt, cinders, stones, glass, metal
particles and other large-sized, relatively non-putrescible organic and inorganic substances.

Grit removal is an essential element of preliminary treatment and its purpose is the
following:

Protects moving mechanical equipment and pumps from abnormal wear due to abrasion
Reduces blockages in pipelines
Prevents sedimentation of materials in aeration tanks and sludge digesters that result in loss
of usable volume. The quantity of grit removed will depend on local circumstances but a rough
indication of the quantity of grit to be expected is 7.5 to 90 m3 M.-1 wastewater treated. The
moisture content of grit is normally in the range 15 to 35%.

De-gritting devices

Hand-Cleaned Channels
These units are generally only used in plants with flows less than 4 Ml/day. The channels have
control devices (venturi flumes or weirs) at their outlets to regulate the velocity to about 0.3
m/s regardless of the wastewater flow. Velocities that are too low allow organics to
contaminate the grit; excessive velocities carry grit to the downstream processes.
At least two elongated channels are provided so that one at a time can be closed off, drained
and the accumulated grit removed manually by shoveling.

Wastewater Treatment
Grit Removal

Mechanically Cleaned Channels

In larger plants, the channels are generally cleaned by a chain-and-flight grit scraper
system without emptying the channels.
Typically, a bucket elevator, inclined screw conveyor or air lift pump removes the grit from a
sump and deposits it into a container or storage hopper for disposal (see sketch below). The
frequency of both the grit scraper and removal systems (manual or automatic) depends on
the rate of grit accumulation.

Cyclone Degritters

Cyclone degritters use centrifugal force in a cone-shaped unit to separate grit from the
wastewater. At a controlled rate, a pump discharges a slurry of grit and organics
tangentially near the upper perimeter of the degritter. This feed velocity creates a vortex that
produces a grit slurry at the lower orifice and a degritted overflow near the top of the unit. In
some systems, a mechanical mixer induces the centrifugal effect.

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment

Biological Filters

Background

One of the earlier methods of treating sewage was to run it into a tank filled with loose stone.
After some 12 hours contact, the sewage was drained away and the stone left in contact with
air. After a period of time a biological slime grew on the stone, which affected the oxidation of
the sewage. These units were known as contact filters. A modification to convert them to
continuous operation, rather than batch, was made in 1893. The sewage was distributed across
the surface of the contact bed by means of a moving mechanism. This was to prevent all the
sewage falling on one spot and short-circuiting the greater mass of stone. A biological filter
(shown in the sketch below), also known as tricking or percolating filter, was developed from
this concept.

Description

The filter consists of an outer shell normally made of concrete. Under-drains on the floor of
the filter allow for the collection of the effluent from the bottom of the filter and allow for the
free passage of air through the filter, which is important, as the micro-organisms, which grow
in a filter, require oxygen from the air to live.
The shell is filled with filter media, the most common being crushed stone which has been
carefully graded. The size of stone commonly varies from 25 - 100 mm in diameter. The
depth of the media also varies from 900 - 4 000 mm (a typical average is approximately 1 800
mm).

Wastewater Treatment

Biological Filters

The filter media can also consist of coke, gravel, blast furnace slag, broken bricks or artificial
media. The filter media should be weather resistant and not dissolve in the sewage.

The flow to the filter (from the primary sedimentation tanks) can be supplied by pumps or by
gravity feed, controlled by a siphon. In the siphon system, during periods of low flow, no water
passes directly to the filters but builds up in the dosing tank until a certain head has been
reached. An air seal on the outlet siphon is then dislodged and the contents of the dosing
tank are discharged to the filter. When the contents of the dosing tank have been discharged,
the air seal on the siphon mechanism is automatically closed.

It is important in any distribution system that every portion of the media receives the same
quantity of flow. In the case of circular filters, this means that distributor arms must be
constructed to permit proportionately increasing amounts of effluent to be discharged from
the inner pivot point to the outer circumference of the filter. Rotation is achieved by forcing
water through the distribution holes and pushing the freely moving pipe away from the falling
water. The available head of water and the size of the holes in the distribution pipe control the
speed of rotation.

Principle of biological filtration

This is an aerobic process and distinct from sludge digestion, which is an anaerobic process.
Sewage is a suitable source of food for the microorganisms in a biological filter as it contains
nitrogen, organic carbon compounds, phosphorus and trace elements. Air circulates in the
voids between the media, taking oxygen to the slime layer on the surface of the media. As
sewage trickles over the media, the various organic substances are absorbed onto the
biological film thus supplying the organisms with food. Oxygen, which is present in the voids
of the filter, dissolves in the water and is then transported to the slime layer. Metabolism of
the substrate then takes place. If either food or oxygen is absent, this metabolism will stop.
End products, which result from metabolism, are mainly water and carbon dioxide, which are
liberated from the slime layer back into the main water flow. This process is most efficient
when the slime layer is thin and totally aerobic.

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment
Biological Filters – troubleshooting guide

Wastewater Treatment
Biological Filters – troubleshooting guide

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment

Activated Sludge – Overview

What is activated sludge?

The process was discovered by the aeration of holding tanks for distributing raw sewage onto
land. It was noticed that the nature of the sewage improved during aeration, which was
applied mainly to prevent odours from forming. This improvement was even more marked
when some of the sludge that was suspended and settled to the bottom during decanting,
was re-suspended during aeration of the following batch of sewage. This led to the Fill and
Draw method of treatment by which the sludge was allowed to settle to the bottom before
decanting the effluent, filling the tank again with raw sewage, re-suspending the sludge by
aeration and repeating the decanting process. It was noticed that under these conditions the
sludge became more active and this process was referred to as activating the sludge. The basic
layout of an activated sludge plant is illustrated in the sketch below. The aeration basin is
followed by a clarifier, where the active sludge is separated from the liquid and returned
(pumped) to the aeration basin, together with the raw influent. The aeration basin or reactor,
the clarifier and return sludge pumping form integral parts of an activated sludge system.

The wastewater, containing numerous organic compounds, serves as a food source for micro-
organisms in the mixture of activated sludge. Air is supplied for the respiration or breathing of
these organisms and also for keeping the organisms in suspension and in contact with the
food source. The organisms use the food to obtain energy, thereby growing to form new
micro-organisms, carbon dioxide and water. The mass of organisms is constantly passed to the
clarifier to be separated by settling and recycled by pumping back to the aeration basin (return
activated sludge – RAS). The surplus sludge (waste activated sludge – WAS) formed by the
additional growth of organisms must be removed from the system to keep the total mass of
organisms constant.

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment

Sludge Digestion - Overview

Reasons for digesting sludge

In wastewater treatment considerable quantities of sludge are constantly being produced. In
their raw state the sludge are putrescible and rapidly develop strong and obnoxious odours.
Sludge also contain pathogenic (disease-causing) organisms harmful to man. It is, therefore,
necessary to contain and treat these wastes.
Anaerobic digestion is the most common method in use today for treating wastewater
sludge. Its attractiveness comes from it being a relatively stable process if properly
controlled, with low operating costs and the production of a useful by-product (a
combustible gas), which can be used as a source of energy.

Advantages

The organic content of the sludge is significantly reduced by conversion into gaseous end
products. This means the sludge becomes stabilised after digestion.
The obnoxious odour of the sludge is removed. The final digested sludge has a
characteristic "tarry" odour.
Fats and grease are broken down by the process.
There is a significant reduction in the numbers of pathogenic bacteria.
There is a marked physical change after digestion due to a significant reduction of the mass
and volume of the sludge.
 There is a marked chemical change after digestion. The liquid fraction (supernatant)
contains increased levels of ammonia as a result of the breakdown of organic nitrogen
(proteins). This makes the digested sludge liquor potentially suitable for agricultural use.
The digested sludge can be readily dewatered and dried.
The biogas, which is evolved, is a mixture of carbon dioxide (CO2) and methane (CH4),
which can be used for digester heating or to generate power.

Wastewater Treatment

Sludge Digestion - Overview

Disadvantages

A relatively high initial capital cost is involved, which tends to limit the process to medium
and large size works.
The slow rate of bacterial growth requires long periods of time for start-up and limits the
flexibility of the process to adjust to changing feed loads, temperatures and other
environmental conditions.
The process is prone to upsets if not regularly monitored and if corrective action is not taken
timeously.

Microbiology of the process

Anaerobic digestion is a multistage biological process whereby bacteria, in the absence of
oxygen, decompose organic matter to carbon dioxide, methane and water. In this way the
sludge is stabilised and the obnoxious odour is removed. The process can, however, be
described adequately and simply as occurring in two stages, involving two different types of
bacteria.

Stage 1 The organic material present in the feed sludge is converted into organic
acids (also called volatile fatty acids) by acid-forming bacteria

Stage 2 These organic acids serve as the substrate (food) for the strictly anaerobic
methane-producing bacteria, which convert the acids into ethane and carbon
dioxide.

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment

Sludge Thickening and Sludge Handling

Sludge thickening?

Thickening is the process used to increase the solids content of sludge by the separation and
removal of a portion of the liquid phase. Gravity thickening makes use of the force of gravity
as the main agent in the settling and thickening process.
The thickening of sludge plays an important role in reducing capital costs relating to the
provision of sludge handling equipment and the operational costs of the handling and
treatment of the sludge. What is not generally realised is that, both in terms of capital outlay
and operating cost, sludge handling usually accounts for over 50% of overall treatment costs.

Methods

There are three accepted methods used for pre-digestion sludge thickening:

Gravity thickening
Dissolved air flotation thickening
Centrifugation

Gravity thickening is the method most commonly used in South Africa

Reasons for thickening sludge before anaerobic digestion

The concentration of thickened solids should be high enough to promote effective
digestion, but not too thick to adversely affect pumping and mixing of the sludge in the
digester. The main reasons for thickening sludge prior to digestion are:

•To maximise the use of the available digester capacity in the digestion of the solids, i.e.
excess water uses up digester capacity and reduces retention time.

•To prevent the dilution of the feed material which would cause difficulty in the utilisation of
the food by the bacteria.

•To reduce the amount of heat required in a heated digester, i.e. excess water with the
sludge also requires heating in order to keep the digester contents at the required
temperature.

Wastewater Treatment
Sludge Thickening and Sludge Handling

To prevent the washout of solids and micro-organisms from a hydraulically overloaded
digester.
To prevent the dilution of the generated alkaline buffer in the digester as this could cause pH
instability.

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment
Sludge Thickening –troubleshooting guide for gravity thickeners

Wastewater Treatment
Sludge Thickening –troubleshooting guide for gravity thickeners

Ref.1. Water Institute of Southern Africa (2002):Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.

Wastewater Treatment
Effluent – Disinfection oveview

Untreated human waste contains all the known human pathogens and is also referred to as
raw sewage, which is considered to be hazardous. Raw sewage contains the following
pathogens3:

Every waste water treatment process reduces the number of micro-organisms3. However,
even after the best possible biological treatment, there still remain high numbers of
pathogenic micro-organisms in the treated wastewater. To render the waste water
effluent safe for discharge into a public stream or river, some additional treatment, such as
disinfection, is required.

Why disinfection?

The disinfection treatment process will remove or inactivate pathogenic
micro-organisms. Not all organisms are killed during disinfection, as is the case with
water sterilization. The purpose of disinfection is to kill or inactivate all the primary
micro-organisms, such as bacteria, viruses and protozoan cysts.

Wastewater Treatment
Effluent – Disinfection oveview

There are many methods for disinfecting wastewater effluents, such as:

Natural processes (predation and normal death).
Environmental factors (salinity and solar radiation).
Methods with certain industrial applications (ultrasound and heat).

The general methods best used for wastewater disinfection includes:

Chlorine disinfection
Bromine, bromine chloride and iodine
Ozone
Ultraviolet radiation

Water Institute of Southern Africa (2002): Handbook for the operation of waste water treatment plants. ISBN 0-958-45346-2.
2. Salmonella food poisoning. http://www.healthline.com/galecontent/salmonella-food-poisoning-1
3. Metcalf and Eddy, Inc. (1991). Wastewater Engineering: Treatment, disposal and re-use. Third Edition, ISBN 0-07-100824-1.McGraw-
Hill, New York. pp. 1334.



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