Wastewater Treatment: Characteristics and Systems

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Wastewater Treatment Characteristics and Systems
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DISPOSAL OF WASTE MATERIAL ON LAND AND WATER
BODIES

• Liquid wastes may be disposed of in a number of
  ways (before giving at least secondary level
  treatment).
• Surface waters (Rivers, Lakes etc)
• On land
• DISPOSAL IN SURFACE WATERS
• In natural streams, there is a balance between
  plant and animal life, with considerable
  interaction among the various life forms. Waters
  of good quality are characterize by multiplicity
  of species with no dominance.

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• Organic matter which enters the stream is broken
  down by bacteria to ammonia, nitrates, sulfates,
  carbon dioxide etc, which are used by plants and
  algae to produce carbohydrates and oxygen.

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Water Pollution

• Introduction of excessive quantities of waster
  material can upset the cycle by causing rapid
  bacterial growth and resulting depletion of
  dissolved oxygen in the stream. Polluted waters
  are characterized by very large number of
  relatively few species.

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Disposal of Wastewater

• Disposal of wastewater in a stream should be
  thus regulated with respect to both quantity and
  concentration in order to safeguard the aquatic
  life and desirable water use. Thus there is a
  limit on the amount of liquid wastewater that can
  be disposed of in a water body, which is called
  the assimilative capacity of that water body.

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ASSIMILATIVE CAPACITY

• Assimilative capacity can thus be defined as the
  amount of wastewater that can be disposed of in
  the water body and it can be safely stabilized
  while maintaining the desired water quality.
• Since a certain amount of wastewater can be
  discharged into a receiving water body, it may be
  highly un-economical to outlaw the wastewater
  discharge. However, excessive discharge will
  impair the stream water quality.

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ON-LAND DISPOSAL OF WASTEWATER

• A treatment, at least to secondary level must be
  given prior to LAND DISPOSAL this is necessary
  due to the following reasons.
• To reduce stress upon SOIL SYSTEM
• To reduce production of NUISANCE
• CONDITIONS.
• Following methods may be employed for land
  disposal of wastewater.
• SPRAY IRRIGATION
• RAPID INFILTRATION
• OVERLAND RUNOFF

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Reuse of Treated Wastewater

• 1. Spray Irrigation Treated sewage or waste
  water may be applied to both forests and
  agricultural lands. Care is however to be taken
  that the treated sewage MEET THE REQUIRED
  COLIFRIM AND BOD STANDARDS as employed for
  various irrigations uses.

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Reuse of Treated Wastewater

• Rapid Infiltration. This is done either for
  Waste water disposal (e.g Soakage pits)
• Ground water recharge
• For GROUND WATER RECHARGE, wastewater is
  discharged into large basins UNDERLAIN BY SAND
  and soil of high permeability. The bottom of the
  basins is covered by BERMUDA GRASS to absorb
  NUTRIENTS.

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Reuse of Treated Wastewater

• Overland Runoff / Flow
• It is not a true disposal system since the
  wastewater must be collected after passage over
  soil. This is in fact a method of TERTIARY
  TREATMENT of wastewater to further reduce its BOD
  and nutrient levels. The grasses are planted on
  the ground over which sewage flow.

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USE OF TREATED SEWAGE FOR IRRIGATION

• All the HUMAN and Animals manure, which the world
  loses by discharging of sewage in to the RIVERS
  if returned to the land instead of being thrown
  into the sea, should suffice to nourish the
  world.
• (VICTOR HUGO 1868)

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Advantages of Wastewater Irrigation

• Use of wastewater for irrigations has following
  advantages.
• Prevention of river pollution and protection of
  surface water quality.
• Conservation of water and nutrients to improve
  agriculture in arid and semi-arid regions.

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HISTORY

• Use of raw wastewater for irrigation purpose can
  be traced back to 1880s. U.K, France, Germany,
  Australia, Mexico practiced it. However in the
  beginning little considerations went into health
  hazards related with raw sewage irrigation
  containing pathogenic and parasitic organisms.
•   After 1945, standards were set for the first
  time for wastewater to be used for irrigation.
  The interest in wastewater reuse gained momentum
  and in many countries concrete efforts were made
  in this direction e.g.

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Examples of Wastewater Reuse

• Khartoum 2800 ha greenbelt was irrigated with
  treated wastewater.
• Mexico City 100,000 ha grain and fodder
  irrigated with treated wastewater.

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Scenario of Wastewater Irrigation in Pakistan

• In Pakistan, raw wastewater is used to irrigate
  800 ha, 2000 ha and 2500 ha land in Lahore,
  Hyderabad and Faisalabad respectively.
• Although wastewater reuse has been practical
  more widely in developing countries over the past
  30 years, much of it is UNPLANNED and
  UNCONTROLLED and possess a threat to public
  health. These risks must be fully understood and
  appropriate measures taken to provide TECHNICALLY
  FEASIBLE and ECONOMICALLY ATTRACTIVE solutions to
  that public can reap the full benefit of
  wastewater reuse without suffering harmful
  effects.

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PUBLIC HEALTH RISK ASSOCIATED WITH RAW SEWAGE
WASTE WATER

• The wastewater stream of a community carries full
  spectrum of pathogenic microorganisms excreted in
  the feces and urine of infected individuals.
  Their concentration is
• Many millions / liter for bacteria
• Thousands / liter for viruses
• Few hundred / liter for helminth eggs
• Most excreted pathogens can survive if the
  environment long enough to be transported by the
  wastewater to the fields.

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Health risk

• Health risk is high for people using salads and
  vegetables EATEN UNCOOKED and irrigated with RAW
  SEWAGE. Such people are exposed to following
  diseases arranged in descending order of their
  chance of occurrence
• Helminthes (worm) disease
• Cholera
• Typhoid
• Similarly SEWAGE FARM WORKERS are also exposed to
  above diseases. However, evidence of bacterial
  and viral diseases among them is limited.
• There is no demonstrated risk to people close to
  sewage-irrigated sites.

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Effective wastewater treatment

• Wastewater treatment process that effectively
  remove all, or most pathogens reduce the negative
  health effects caused by the utilization of
  wastewater for irrigation

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ASPECT OF WASTE WATER REUSE

• Wastewater from a city of 500,000 persons with
  175-lpcd water consumption can irrigate 2700 ha
  of land. wastewater effluent has a SIGNIFICANT
  FERTILIZER VALUE. It can supply all or more of
  the NITROGEN and much of the PHOSPHORUS and
  POTASSIUM required for agriculture crops.
• Furthermore, the organic matter in the effluent
  adds valuable MICRONUTRIENTS and HUMAS to the
  soil, which helps to improve WATER RETENTION
  capacity of soil.

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Advantages of wastewater irrigation

• Studies in California, Portugal and Israel have
  shown that many crops can thrive under wastewater
  irrigation without any additional chemical
  fertilizer.

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W.H.O GUIDELINES

• Parameter Value
• Helminth Eggs 1 / liter
• Fecal Coliform 1000 / 100 ml
• for irrigation of crop likely to be eaten
  uncooked, sports fields, public parks
• Mmicronutrients-
• Very small quantity of certain substances are
  required for crops. E.g. Mn, Fe etc.

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Significance of Wastewater Contaminants

• Suspended solids can cause sludge deposits and
  anaerobic conditions in the environment
• Biodegradable organics can cause anaerobic
  conditions in the environment
• Pathogens transmit disease
• Nutrients can cause eutrophication
• Heavy metals toxicity to biota and humans
• Refractory organics toxicity to biota and
  humans
• Dissolved solids interfere with reuse

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Characteristics of Domestic Wastewater
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On-Site Disposal Systems

• In locations where sewers and a centralized
  wastewater treatment system are not available, on
  site disposal must be used
• Septic systems most common for individual
  residences
• Engineered systems used for unfavorable site
  conditions
• Larger systems required for housing clusters,
  rest areas, commercial and industrial facilities

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Septic Systems
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Septic Systems
Septic Tank settling, flotation and anaerobic
degradation
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Septic Systems
Drain field (cross-section) aerobic degradation
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Septic Systems

• Soil must pass percolation test
• soil type
• rate of water infiltration
• depth to water table
• Design specifications
• Tank volume and number of chambers
• Drain field size
• Drain field materials
• Basis for design is empirical
• Tank must be pumped to remove solids every 1-3
  years
• Drain field replacement may be required

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Engineered Systems
Mound System
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Engineered Systems

• Intermittent sand filter can be designed for
• pulsed dosing
• even distribution
• high treatment efficiency
• leakage protection

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Municipal Wastewater Treatment Systems

• Pretreatment removes materials that can cause
  operational problems, equalization optional
• Primary treatment remove 60 of solids and
  35 of BOD
• Secondary treatment remove 85 of BOD and
  solids
• Advanced treatment varies 95 of BOD and
  solids, N, P

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Pretreatment of Industrial Wastewaters

• Industrial wastewaters must be pretreated prior
  to being discharged to municipal sewer system
• Approach is to remove materials that will not be
  treated by municipal system
• Local authority must monitor and regulate
  industrial discharges
• Pretreatment requirements set by U.S. EPA or by
  any other monitoring agency

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Bar racks

• Purpose
• remove larger objects
• Solid material stored in hopper and sent to
  landfill
• Mechanically or manually cleaned

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Grit Chambers

• Purpose remove inert dense material, such as
  sand, broken glass, silt and pebbles
• Avoid abrasion of pumps and other mechanical
  devices
• Material is called grit

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Grit Chambers Velocity Controlled
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Type-I Settling -- Stokes Law

• where
• ?s settling velocity
• ?s density of particle (kg/m3)
• ? density of fluid (kg/m3)
• g gravitational constant (m/s2)
• d particle diameter (m)
• µ dynamic viscosity (Pas)

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Example Grit Chamber Design

• Design a grit chamber to remove sand particles
  (?p 2650 kg/m3) with a mean diameter of 0.21
  mm. Assume the sand is spherical and the
  temperature of the wastewater is 20 oC. The
  wastewater flow is 10,000 m3/d. A velocity of
  0.3 m/s will be automatically maintained, and the
  depth must be 1.5 times the width at maximum
  flow.

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Example

• Calculate settling velocity

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Example

• Calculate the cross-sectional area

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Example

• Calculate the width and depth

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Example

• Determine the detention time required for a
  particle to fall the entire tank depth
• Determine the length to achieve this detention
  time

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Example

• Thus, the tank must have dimensions
• W 0.51 m
• D 0.76 m
• L 5.8 m