WASTE WATER TREATMENTPRIMARY AND SECONDARY TREATMENT

ASHIKA RAVEENDRAN 2ND MSC BIOTECHNOLOGY

INTRODUCTION Wastewater treatment consists of applying known technology

to improve or upgrade the quality of a wastewater

Wastewater treatment involves collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes

The principal objective of wastewater treatment is generally to allow human and industrial effluents to be disposed off without danger to human health or unacceptable damage to the natural environment

With the current emphasis on environmental health and water pollution issues, there is an increasing awareness of the need to dispose of these wastewaters safely and beneficially

The indiscriminate discharge of these wastewater streams into the environment can

- Render soils "sick ". - Pollute the receiving bodies of water. - Cause air pollution by generating obnoxious gases.

Discharge untreated wastewater into the domestic sewage system makes the task of treating domestic sewage, a very difficult and costly exercise.

To prevent any health hazards caused by discharging wastewater into the environment and protect domestic sewage , the wastewater must be treated before discharge.

Conventional wastewater treatment processes

Conventional wastewater treatment processes

Preliminary treatment

Primary treatment

Secondary treatment

Tertiary and/or advanced treatment

Important Contaminants Of Concern In Wastewater Treatment

Suspended solids ● Lead to the development of sludge deposits and

anaerobic conditions when untreated wastewater is discharged into the aquatic environment.

Nutrients (P, N2&C) ● When discharged into the aquatic environment, these

nutrients can lead to the growth of undesirable aquatic life. When discharged in excessive amounts on land, they can also lead to the pollution of groundwater.

Priority pollutants ● Organic and inorganic compounds selected on the

basis of their known or suspected carcinogenicity, or high acute toxicity. Many of these compounds are found in wastewater.

Refractory organics ● These organics tend to resist conventional methods of

wastewater treatment. Typical examples include surfactants, phenols, and agricultural pesticides.

Heavy metals ● Heavy metals are usually discharged to wastewater

from commercial and industrial activities and have to be removed if the wastewater is to be reused.

Dissolved inorganics ● Inorganic constituents such as calcium, sodium, and

sulfate are added to the original domestic water supply as a result of water use and may have to be removed if the wastewater is to be reused.

Characteristics Of Industrial Wastewater

1 Physical Characteristics 1.1 Total Solids ● Analytically the total solids content of a

wastewater is defined as all the matter that remains as residue upon evaporation at 103 to 105°C.

● Composed of: 1.Floating matter. 2.Settleable matter. 3.Colloidal matter. 4.Matter in solution.

Classification of total solids

1.2 Odors ● Industrial wastewater may contain either odorous

compounds or compounds that produce odor during the process of wastewater treatment.

1.3 Temperature ● The temperature of water is a very important

parameter because of its effect on - Chemical reactions and reaction rates in

treatment process. - Aquatic life. 1.4 Color ● Color of industrial wastewater varies according to the

type of industry.

● Most colored matter is in a dissolved state..

1.5 Turbidity ● Turbidity, a measure of the light-transmitting

properties of water, is another test used to indicate the quality of wastewater discharges and natural waters with respect to colloidal and residual suspended matter.

2 Chemical Characteristics 2.1 Organic Matter ● The presence of these substances has complicated

industrial wastewater treatment because many of them either cannot be or are very slowly decomposed biologically.

● Typical examples include: - Fats, Oils, and Grease. - Surfactants. - Phenols. - Volatile Organic Compounds (VOCs). - Pesticides & Agricultural Chemicals.

2.1.1 Parameters Of Organic Content 2.1.1.1 Biochemical Oxygen Demand (BOD5) ● The determination of the BOD5 involves the

measurement of the dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter.

2.1.1.2 Chemical Oxygen Demand (COD) ● It oxidizes organic matter through a reaction with a

mixture of chromic and sulfuric acid at high temperatures.

● The COD of wastewater is, in general, higher than that of the BOD because more compounds can be chemically oxidized than can be biologically oxidized.

● This can be very useful because COD can be determined in 3 hours, compared with 5 days for the BOD5.

2.2 Inorganic Matter ● Nitrogen & Phosphorus. ● Sulfur. ● Heavy Metals. 3 Biological Characteristics ● Some industries have certain pathogenic organisms

like slaughterhouses others have molds and fungi as starch and yeast factories.

● Biological information is needed to assess the degree of treatment of the wastewater before its discharge to the environment.

Common Types Of Wastewater Treatment Methods

1 Physical Unit Operations ● Treatment methods in which the application of

physical forces predominates.

● Screening, mixing, flocculation, sedimentation, flotation, filtration, and gas transfer are typical unit operations.

2 Chemical Unit Processes ● Treatment methods in which the removal or

conversion of contaminants is brought about by the addition of chemicals or by other chemical reactions.

● Precipitation, adsorption, and disinfection are the most common examples used in wastewater treatment.

3 Biological Unit Processes ● Treatment methods in which the removal of

contaminants is brought about by biological activity.

● Biological treatment is used primarily to remove the biodegradable organic substances (colloidal or dissolved) and nutrients (nitrogen & phosphorus) from wastewater.

● Basically, these substances are converted into gases that can escape to the atmosphere and into biological cell tissue that can be removed by settling.

Conventional wastewater treatment consists of a combination of physical, chemical and biological processes and operations to remove solids, organic matter and sometimes, nutrients from wastewater.

General treatment level, are preliminary, primary, secondary, and tertiary and/or advanced wastewater treatment.

In some countries, disinfection to remove pathogens sometimes follows the last treatment step

PRELIMINARY TREATMENT The objective of preliminary treatment is the removal of

coarse solids and other large materials often found in raw wastewater

Preliminary treatment operations typically include coarse screening, grit removal and, in some cases, comminution of large objects

In grit chambers, the velocity of the water through the chamber is maintained sufficiently high, or air is used, so as to prevent the settling of most organic solids

Comminutors are sometimes adopted to supplement coarse screening and serve to reduce the size of large particles so that they will be removed in the form of a sludge in subsequent treatment processes

Grit chamber

Aerated grit chamber : diffused air keeps organic solids in suspension as grit settles

comminutor•In this device all of the wastewater flow passes through the grinder assembly

•The grinder consists of a, screen or slotted basket, a rotating or oscillating cutter and a stationary cutter•Solids pass through the screen and are chopped or shredded between the two cutters

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 50% of the incoming biochemical oxygen demand (BOD5), 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

In many industrialized countries, primary treatment is the minimum level of pre-application treatment required for wastewater irrigation

It may be considered sufficient treatment if the wastewater is used to irrigate crops that are not consumed by humans or to irrigate orchards, vineyards, and some processed food crops

Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours

Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units

Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units

Sedimentation tank and clarifiers

Typical sedimentation tanks:

(a)rectangular horizontal flow

tank;

(b)circular, radial-flow tank

(c) hopper-bottomed, upward

flow tank

SECONDARY TREATMENT The objective of secondary treatment is the further

treatment of the effluent from primary treatment to remove the residual organics and suspended solids

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 (principally CO2, NH3, and H2O)

Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter

High-rate biological processes are characterized by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes

Consequently, the growth rate of new organisms is much greater in high-rate systems because of the well controlled environment

The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent

The sedimentation tanks used in secondary treatment, often referred to as secondary clarifiers, operate in the same basic manner as the primary clarifiers described previously

The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing

Common high-rate processes include the activated sludge processes, trickling filters or biofilters, oxidation ditches and rotating biological contactors (RBC)

A combination of two of these processes in series (e.g. biofilter followed by activated sludge) is sometimes used to treat municipal wastewater containing a high concentration of organic material from industrial sources

Activated Sludge 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 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 effluent

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

Sequential Batch Reactor “SBR”

● The unit processes involved in the SBR and conventional activated-sludge systems are identical. Aeration and sedimentation/clarification are carried out in both systems. However, there is one important difference. In conventional plants, the processes are carried out simultaneously in separate tanks, whereas in SBR operation the processes are carried out sequentially in the same tank.

● As currently used, all SBR systems have five steps are commonly carried out in sequence as follows: 1- fill. 2- react (aeration). 3- settle (sedimentation/clarification). 4-draw (decant). 5- idle.

● A unique feature of the SBR system is that there is no need for a return activated-sludge (RAS) system. Because both aeration and settling occur in the same chamber, no sludge is lost in the react step, and none has to be returned from the clarifier to maintain the sludge content in the aeration chamber.

Aerated Lagoon An aerated lagoon or aerated basin is a

holding and/or treatment pond provided with artificial aeration to promote the biological oxidation of waste waters.

There are many other biological processes for treatment of wastewaters, for example activated sludge, trickling filters, rotating biological contactors and biofilters.

They all have in common the use of oxygen (or air) and microbial action to biotreat the pollutants in wastewaters.

Trickling Filter 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

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

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.

PACKED BED REACTOR A reactor in which the filled inert packing

material for the growth of biomass is kept packed or fixed is called a packed bed reactor.

The flow of waste water through the reactor may be upward or downward.

The packing material commonly used is slag, rock or ceramics.

Fixed film reactors Fixed film reactors are biological filters

wherein microorganisms are attached to an inert supporting medium ,which is packed into a tower or tank.

The distribution of effluent is done in a reactor and air is introduced from the bottom vent and passes it across the media bed as effluent percolates or distributes.

By using organic matter and oxygen, microbial slime develops on media support.

When the thickness of the slime increases, extra biomass sloughs off.

This sludge is collected by gravity in a sedimentation tank.

Similar to activated sludge treatment , the sludge is treated and disposed off safely

Fluidized Bed Reactor In fluidized bed reactor the solid material

used is either inert (sand , coal, plastic) or active (granular activated charcoal)

In FBR, the biomass is attached to the surface of small, low specific gravity particles , which are kept in suspension by the upward velocity of liquid flow.

UASB reactor Up flow anaerobic sludge blanket technology also known

as UASB reactor is a form of anaerobic digester which used in wastewater treatment.

UASB reactor is a methane-producing digester, which uses an anaerobic process and forming a blanket of granular sludge and is processed by the anaerobic microorganisms.

UASB reactor is based on the so-called three-phase separator, which enables the reactor to separate gas, water and sludge mixtures under high turbulence conditions. This allows for compact, cheaper designs.

During the treatment process a amount of valuable biogas energy will be produced which can be collected for other usage.

Much less bio-solids waste generated compared with aerobic process because much of the energy in the wastewater is converted to a gaseous form and resulting in very little energy left for new cell growth.

A low energy requirement for the treatment process.

Less nutrients required.

Anaerobic Digester Anaerobic digesters are large fermentation tanks

continuously operated under anaerobic conditions.

Used for direct treatment of high BOD effluents.

Mechanisms : mechanical mixing , heating , gas collection , sludge addition, removal of stabilized sludge.

Digester have large variety of non-methanogenic and obligatory or facultative anaerobic bacteria.

Complex organic material is broken down into methane (70%) and carbon dioxide(30%)

facts•Asia’s Largest STP is at Bharwara situated at outskirts of Lucknow Uttar Pradesh.

•It treats 340 million litres of the city’s 400 million of discharge each day

•Deer Island STP in Boston produces 3mv of electricity using methane evolved from the digester.

•It also uses the sludge as fertilizers and produces 75 tons of fertilizer each day.

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