Docstoc

wastewater+treatment

Document Sample
wastewater+treatment Powered By Docstoc
					                                  Wastewater Treatment
Common Wastewater Terms

Activated Sludge
The term activated sludge refers to the brownish flocculent culture of organisms developed in an aeration
tank under controlled conditions. Also, sludge floc produced in raw or settled wastewater by the growth of
zoological bacteria and other organisms in the presence of dissolved oxygen. A good quality of activated
sludge is shown by brown color, good settling characteristics, and DO present.

Alkalinity
The capacity of water to neutralize acids, a property imparted by the water's content of carbonates,
bicarbonates, hydroxides, and occasionally borates, silicates, and phosphates.

Anaerobic
A biological environment that is deficient in all forms of oxygen, especially molecular oxygen, nitrates and
nitrites. The decomposition by microorganisms of waste organic matter in wastewater in the absence of
dissolved oxygen is classed as anaerobic.

Anoxic
A biological environment that is deficient in molecular oxygen, but may contain chemically bound oxygen,
such as nitrates and nitrites.

Bacteria
Bacteria are living organisms (animals) that cannot be seen by the naked eye. They are a group of
universally distributed, rigid, essentially unicellular, microscopic organisms lacking chlorophyll. They are
characterized as spheroids, rod-like, or curved entities, but occasionally appearing as sheets, chains, or
branched filaments.

Biochemical Oxygen Demand (BOD)
The BOD test is used to measure the strength of wastewater. The BOD of wastewater determines the
milligrams per liter of oxygen required during stabilization of decomposable organic matter by aerobic
bacteria action. Also, the total milligrams of oxygen required over a five-day test period to biologically
assimilate the organic contaminants in one liter of wastewater maintained at 20 degrees Centigrade.

Bulking Sludge
A phenomenon that occurs in activated sludge plants whereby the sludge occupies excessive volumes and
will not concentrate readily. This condition refers to a decrease in the ability of the sludge to settle and
consequent loss over the settling tank weir. Bulking in activated sludge aeration tanks is caused mainly by
excess suspended solids (SS) content. Sludge bulking in the final settling tank of an activated sludge plant
may be caused by improper balance of the BOD load, SS concentration in the mixed liquor, or the amount
of air used in aeration.

Chemical Oxygen Demand (COD)
The milligrams of oxygen required to chemically oxidize the organic contaminants in one liter of
wastewater.

Composite Sample
To have significant meaning, samples for laboratory tests on wastewater should be representative of the
wastewater. The best method of sampling is proportional composite sampling over several hours during the
day. Composite samples are collected because the flow and characteristics of the wastewater are
continually changing. A composite sample will give a representative analysis of the wastewater conditions.

Denitrification
A biological process by which nitrate is converted to nitrogen gas.



                                                                                                          1
Digestion
The biological decomposition of organic matter in sludge resulting in partial gasification, liquefaction, and
mineralization of putrescible and offensive solids.

Disinfection
The killing of pathogenic organisms is called disinfection.

Dissolved Oxygen (DO)
The oxygen dissolved in water, wastewater, or other liquid. DO is measured in milligrams per liter. If the
DO of a sample of water is 2 mg/L, it means that there are 2lbs of oxygen in 1 mil lb of water.


Dissolved Solids
Solids that cannot be removed by filtering are dissolved solids.

Extended Aeration
A modification of the activated sludge process which provides for aerobic sludge digestion within the
aeration system.

Floc
Clumps of bacteria and particles that have come together to form clusters, or small gelatinous masses. The
floc mass in an activated sludge aeration tank generally consists of microorganisms.

Grease
In wastewater, a group of substances, including fats, waxes, free fatty acids, calcium and magnesium soaps,
mineral oils, and certain other non-fatty materials.

Milligrams per Liter (mg/L)
A unit of concentration of water or wastewater constituent. It is 0.001 g of the constituent in 1000 ml of
water. The unit parts per million is identical to milligrams per liter.

Mixed Liquor (ML)
The mixture of activated sludge, wastewater, and oxygen, wherein biological assimilation occurs.

Mixed Liquor Suspended Solids (MLSS)
The milligrams of suspended solids per liter of mixed liquor that are combustible at 550 degrees
Centigrade. An estimate of the quantity of MLSS to be wasted from the aeration tank of an extended
aeration plant may be determined by the rate of settling and centrifuge tests on the sludge solids.

Nitrification
The conversion of nitrogen matter into nitrates by bacteria. Nitrogen > Nitrogen is present in wastewater in
many forms: total Kjeldahl nitrogen, ammonia nitrogen, organic nitrogen.

Nitrogen Cycle
The cycle of life, death, and decay involving organic nitrogenous matter is known as the nitrogen cycle. In
the nitrogen cycle ammonia is produced from proteins.

Orthophosphate
A simple compound of phosphorous and oxygen that is soluble in water.

Oxic
A biological environment which contains molecular oxygen; aerobic.

Polyphosphate
A large compound formed of several orthophosphate molecules connected by phosphate-storing
microorganisms.


                                                                                                           2
Raw Wastewater
Wastewater before it receives any treatment. Reactor A tank where a wastewater stream is mixed with
bacterial sludge and biochemical reactions occur.

Return Sludge
Settled activated sludge returned to mix with incoming raw or primary settled wastewater. When the return
sludge rate in the activated sludge process is too low, there will be insufficient organisms to meet the waste
load entering the aerator.

Return Activated Sludge
Activated return sludge is normally returned continuously to the aeration tank. Recycling of activated
sludge back to the aeration tank provides bacteria for incoming wastewater. Its should be brown in color
with no obnoxious odor and is often also returned in small portions to the primary settling tanks to aid
sedimentation. Settled activated sludge is generally thinner than raw sludge. Some activated sludge will be
wasted to prevent excessive solids build up.

Sludge Age
In the activated sludge process, a measure of the length of time a particle of suspended solids has been
undergoing aeration, expressed in day. It is usually computed by dividing the weight of the suspended
solids in the aeration tank by the weight of excess activated sludge discharged from the system per day.



Sludge Digestion
The purpose of sludge digestion is to separate the liquid from the solids to facilitate drying. The proper pH
range for digested sludge is 6.8 - 7.2. Sludge Index Properly called sludge volume index (SVI). It is the
volume in millimeters occupied by 1 g of activated sludge after settling of the aerated liquid for 30 minutes.

Sludge Reaeration
The continuous aeration of sludge after initial aeration for the purpose of improving or maintaining its
condition.

Splitter Box
A division box that splits the incoming flow into two or more streams. A device for splitting and directing
discharge from the head box to two separate points of application.

Wastewater
Domestic wastewater is 99.9% water and 0.1% solids. Fresh wastewater is usually slightly alkaline. If the
pH of the raw wastewater is 8.0, it indicates that the sample is alkaline. If wastewater has a pH value of 6.5,
it means that it is acid. Wastewater is said to be septic when it is undergoing decomposition.


What is wastewater, and why treat it?

         We consider wastewater treatment as a water use because it is so interconnected with the other
uses of water. Much of the water used by homes, industries, and businesses must be treated before it is
released back to the environment.

         If the term "wastewater treatment" is confusing to you, you might think of it as "sewage
treatment." Nature has an amazing ability to cope with small amounts of water wastes and pollution, but it
would be overwhelmed if we didn't treat the billions of gallons of wastewater and sewage produced every
day before releasing it back to the environment. Treatment plants reduce pollutants in wastewater to a level
nature can handle.




                                                                                                             3
        Wastewater is used water. It includes substances such as human waste, food scraps, oils, soaps and
chemicals. In homes, this includes water from sinks, showers, bathtubs, toilets, washing machines and
dishwashers. Businesses and industries also contribute their share of used water that must be cleaned.

          Wastewater also includes storm runoff. Although some people assume that the rain that runs down
the street during a storm is fairly clean, it isn't. Harmful substances that wash off roads, parking lots, and
rooftops can harm our rivers and lakes.

Why Treat Wastewater?

        It's a matter of caring for our environment and for our own health. There are a lot of good reasons
why keeping our water clean is an important priority:

Fisheries
         Clean water is critical to plants and animals that live in water. This is important to the fishing
industry, sport fishing enthusiasts, and future generations.


Habitats
          Our rivers and ocean waters teem with life that depends on shoreline, beaches and marshes. They
are critical habitats for hundreds of species of fish and other aquatic life. Migratory water birds use the
areas for resting and feeding.

Wildlife Recreation and Quality of Life

          Water is a great playground for us all. The scenic and recreational values of our waters are reasons
many people choose to live where they do. Visitors are drawn to water activities such as swimming,
fishing, boating and picnicking.

Health Concerns
         If it is not properly cleaned, water can carry disease. Since we live, work and play so close to
water, harmful bacteria have to be removed to make water safe.

         The major aim of wastewater treatment is to remove as much of the suspended solids as possible
before the remaining water, called effluent, is discharged back to the environment. As solid material
decays, it uses up oxygen, which is needed by the plants and animals living in the water. "Primary
treatment" removes about 60 percent of suspended solids from wastewater. This treatment also involves
aerating (stirring up) the wastewater, to put oxygen back in. Secondary treatment removes more than 90
percent of suspended solids.


Wastewater Treatment Plant: Primary Treatment of Wastewater




The Primary Treatment Process




                                                                                                              4
    1.   Screening: Wastewater entering the treatment plant includes items like wood, rocks, and even
         dead animals. Unless they are removed, they could cause problems later in the treatment process.
         Most of these materials are sent to a landfill.

    2.   Pumping: The wastewater system relies on the force of gravity to move sewage from your home
         to the treatment plant. So wastewater-treatment plants are located on low ground, often near a
         river into which treated water can be released. If the plant is built above the ground level, the
         wastewater has to be pumped up to the aeration tanks (item 3). From here on, gravity takes over to
         move the wastewater through the treatment process.

    3.   Aerating: One of the first steps that a water treatment facility can do is to just shake up the
         sewage and expose it to air. This causes some of the dissolved gases (such as hydrogen sulfide,
         which smells like rotten eggs) that taste and smell bad to be released from the water. Wastewater
         enters a series of long, parallel concrete tanks. Each tank is divided into two sections. In the first
         section, air is pumped through the water.

         As organic matter decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygen
         through the water also keeps the organic material suspended while it forces 'grit' (coffeegrounds,
         sand and other small, dense particles) to settle out. Grit is pumped out of the tanks and taken to
         landfills

    4.   Removing sludge Wastewater then enters the second section or sedimentation tanks. Here, the
         sludge (the organic portion of the sewage) settles out of the wastewater and is pumped out of the
         tanks. Some of the water is removed in a step called thickening and then the sludge is processed in
         large tanks called digesters.

    5.   Removing scum: As sludge is settling to the bottom of the sedimentation tanks, lighter materials
         are floating to the surface. This 'scum' includes grease, oils, plastics, and soap. Slow-moving rakes
         skim the scum off the surface of the wastewater. Scum is thickened and pumped to the digesters
         along with the sludge.

         Many cities also use filtration in sewage treatment. After the solids are removed, the liquid sewage
         is filtered through a substance, usually sand, by the action of gravity. This method gets rid of
         almost all bacteria, reduces turbidity and color, removes odors, reduces the amount of iron, and
         removes most other solid particles that remained in the water. Water is sometimes filtered through
         carbon particles, which removes organic particles. This method is used in some homes, too.

    6.   Killing bacteria: Finally, the wastewater flows into a 'chlorine contact' tank, where the chemical
         chlorine is added to kill bacteria, which could pose a health risk, just as is done in swimming
         pools. The chlorine is mostly eliminated as the bacteria are destroyed, but sometimes it must be
         neutralized by adding other chemicals. This protects fish and other marine organisms, which can
         be harmed by the smallest amounts of chlorine.

         The treated water (called effluent) is then discharged to a local river or the ocean.

Wastewater Residuals:
         Another part of treating wastewater is dealing with the solid-waste material. These solids are kept
for 20 to 30 days in large, heated and enclosed tanks called 'digesters.' Here, bacteria break down (digest)
the material, reducing its volume, odors, and getting rid of organisms that can cause disease. The finished
product is mainly sent to landfills, but sometimes can be used as fertilizer.


Reclaimed Wastewater: Using Treated Wastewater for Other Purposes

    The use of reclaimed wastewater helps us in two ways:
    1. Reclaimed water can supply needed water for some purposes.


                                                                                                             5
    2.   Reclaimed wastewater frees up fresh water that can be used somewhere else, such as for drinking
         water

       California is a good place to go to see how reclaimed wastewater is being used. The East Bay
Municipal Utility District has a working water reclamation project that benefits the community in these
ways:
          Conserves drinking water
          Reduces pollution into San Francisco Bay
          Provides water for irrigation and industrial purposes

        Their project results in a savings of about 5.5 billion gallons per year. Eventually the project will
save enough water to provide drinking water to 83,000 households.

          So, what exactly is reclaimed wastewater used for? A lot of it goes toward watering golf courses
and landscaping alongside public roads, etc. Some industries, such as power-generation plants can use
reclaimed wastewater. A lot of water is needed to cool power-generation equipment, and using wastewater
for this purposes means that the facility won't have to use higher-quality water that is best used somewhere
else.




                                                                                                           6
                                 Wastewater Treatment Outline
I. INTRODUCTION

A. GENERAL
     Municipal sewage is the liquid waste from homes, businesses, and industries.
     It contains water, organic matter, mineral nutrients, microorganisms, and pathogens.
     It is usually treated and released into a river, lake, or ocean (receiving water).

B. REASONS FOR TREATMENT
     For ecological and public health reasons, the impurities must be removed prior to release.
     Pathogens must be removed for public health.
     Organic matter and nutrients must be removed to prevent degradation of the receiving water.

C. RAW SEWAGE
     Municipal sewage comes primarily from toilets, bath, laundry, dishwashers, garbage disposals, car
      wash.
     About 150-200 gal/day/person, mostly water.
     99.9% water.
     0.1% waste

D. SEWERAGE
     Household wastewater is delivered by the sanitary sewer system to the wastewater treatment
      facility.
     Gravity driven with occasional lift stations.

E. STORM DRAINS
     In modern systems storm water runoff and domestic sewage are kept separate.
     Storm water is collected by storm drains.
     Mixing the two results in overloaded STP during storms and release of raw sewage.

F. INDUSTRIAL WASTES
      Industries may also contribute to municipal sewage after on-site pretreatment.
      Or they can release directly to receiving water after complete treatment
      Clean Water Act prohibits negative impact of industrial waste on either the STP or the receiving
       water
      Regulatory agencies monitor compliance

G. WASTEWATER TREATMENT PLANT
     Domestic sewage delivered to plant by sanitary sewer system.
     Purpose is to reclaim water by removing impurities
     Sewage treatment plant = wastewater treatment plant = water pollution control facility
     Engineered to operate efficiently, effectively, and inexpensively
     Takes advantage of simple inexpensive physical (Stokes Law and gravity) and biological (aerobic
      respiration) processes to accomplish its goal

H. WATER PROTECTION LEGISLATION
     Pollution control laws
     Clean Water Act of 2004


II. COMPOSITION OF DOMESTIC SEWAGE

A. WATER
     raw sewage is 99.9% water
     this water must be restored to near its original condition


                                                                                                          7
      recovery of water is the purpose of the wastewater treatment process

B. GRIT AND DEBRIS

       1. Grit
            sand and gravel
            inadvertently introduced to the sewer system through cracks and leaks
            prior to separation of storm drains and sanitary sewers it was abundant
       2. Debris
            plastic bags, rags, some paper (not toilet paper), sanitary napkins, condoms
            this is stuff you should not flush down the toilet
            (toilet paper disintegrates into its component fibers)

C. PARTICULATE ORGANIC MATTER

       1. General
            particles of various sizes
            bacteria (abundant)
            feces, garbage from disposal, disintegrated toilet paper
            contributes to BOD
            contributes to turbidity
            particulate matter will settle in still water

       2. Biochemical Oxygen Demand

          A. General
            BOD
            the amount of oxygen required to oxidize all organic matter to CO 2 and H2O
                       C6H12O6 + O2 -->H20 + CO2
            it is a measure of the amount of organic matter present
            a major STP function is to oxidize the organic matter and remove the BOD
            BOD should be oxidized in the STP rather than in the receiving water

         B. BOD and Receiving Water
             bacteria may exhaust DO by respiring BOD
             results in anoxia, fish kills
             organic matter may accumulate on bottom

         C. Nutrients and the Receiving Water
             nutrients stimulate growth of algae
             algae produce more organic matter and increase BOD

 D. COLLOIDAL AND DISSOLVED ORGANIC MATTER
          much of the organic matter is colloidal or dissolved
          does not settle
          also contributes to BOD and to turbidity

 E. DISSOLVED INORGANIC MATERIAL
           primarily mineral nutrients
           ammonia, nitrate, and phosphate
           from feces, urine, kitchen wastes, laundry (phosphates), car washes

 F. PATHOGENS
          water-borne pathogens are present in domestic sewage
          typhoid, cholera, salmonellosis, diarrhea, infectious hepatitis A, polio, dysentery,
            gairdiasis, and many parasitic worms.


                                                                                                  8
                removal of pathogens prior to release is a major function of the STP
                *a safe drinking water supply is the major reason for the good health enjoyed by
                 developed nations

 G. MISCELLANEOUS
           heavy metals, pesticides, exotic toxins
           illegal materials from homes, businesses, hospitals, schools, laboratories, industries


III. TREATMENT

A. PREVIEW
     objective is removal of impurities from wastewater to reclaim clean water
     process relies heavily on the tendency for particles to settle
     whenever possible soluble materials are converted to particulates and settled
     liquid and solids are separated by settling and treated separately

B. INTRODUCTION
       1. General
            objective is accomplished primarily by inexpensive physical and biological processes
            gravity is used to transport waste whenever possible
            settling (gravity again) is used to remove particulate organic material (POM) from water
            bacterial respiration is used to remove dissolved organic material (DOM) from the water
            in general, particulates are easy to remove, solutes are difficult

        2. Treatment Plant
           a. Input (Raw Wastewater)
              *turbid, malodorous, unsanitary
              *suspension of debris, organisms, pathogens, toxins, minerals, organic material
           b. Output (Finished Wastewater)
              clean, transparent, odorless, colorless, sanitary, safe water with low BOD, and pathogen-
                  free
              ideally with low nutrient levels

        3. Level of Treatment
             there are several levels of treatment
                      o Pre-treatment
                      o Primary treatment
                      o Secondary treatment
                      o Tertiary treatment
                      o Post-treatment

                there are several levels of treatment
                complete treatment, with all stages, removes all four major components (DIM, DOM,
                 POM, and pathogens)
                most STPs in developed nations remove all but DIM
                more and more treatment plants also remove DIM
                much of the third world does not even have collection systems

        4. Overview

C. PRE-TREATMENT
              raw wastewater pumped to top of system and flows downhill through the plant

        1. Objectives
                  removes grit and debris


                                                                                                       9
                    to prevent clogging or damage to valves, pumps, and pipes

        2. Mechanisms
          a. Bar Screen
                  parallel steel bars or grid through which water flows
                  debris trapped on screen is removed and sent to landfill or incinerated

         b. Grit Settling Chamber
                  flow velocity is reduced slightly so that sand and gravel settle
                  the first of several steps that use gravity to separate particles from liquid
                  POM remains suspended
                  settled solids taken to landfill

D. PRIMARY TREATMENT

        1. Objective
             *removal of particulate organic material (POM)
             *this reduces BOD (and nutrients)

        2. Mechanism
           a. Principle
              *POM is allowed to settle in quite water
              *gravity!

          b. Process
             primary clarifiers have low flow (2 m/hour)
             particles settle to bottom (30-50% of POM)
             clarified water flows over the sides
             solids (=raw sludge) are sent to sludge facility (more later)
             clarified water sent to secondary treatment
             *grease and fat skimmed off the top

          c. Output
             waste has been separated into sludge and water
             they will be treated separately

E. SECONDARY TREATMENT
       1. Objective
            removal of dissolved organic material
            this further reduces the BOD (and nutrients)

        2. Principle
              2° treatment is a biological process
              aerobic bacteria respire DOM and convert it to CO 2 and H2O
              C6H12O6 + O2 --> CO2 + H2O
              reduces the BOD in the STP instead of the receiving water

        3. Mechanisms
             several methodologies are available
             all provide microorganisms with abundant oxygen for respiration of DOM

          a. Trickling Filters
                  the oldest of the methodologies
                  large tanks filled with a substrate (medium) that provides abundant surface area
                     exposed to air (oxygen)
                  medium may be rocks or synthetic


                                                                                                      10
                 supernatant from primary clarifiers is applied to the media and allowed to flow over
                  its surfaces
                 bacteria growing on the surfaces respire the DOM
                 biological community with several trophic levels
                 bacteria are the base of the food web
                 no producers; the energy supply is allochthonous (it is DOM from your home)
                 bacteria, protozoa, rotifers, insect larvae
                 insect larvae include rat-tail maggots, filter flies, horseflies
                 these do not contribute importantly to respiration but can be a nuisance
                 filters are periodically flooded to kill insect larvae
                 trickling filter ecosystem lacks producers
                 relies on allochthonous production (BOD)
                 there is a little photosynthesis on the surface layer of rocks (negligible)
                 trickling filters can achieve 85-90% BOD removal

        b. Activated Sludge
                the most widely used system
                more complex, more expensive
                requires less space
                a little more effective
                employs a large tank through which clarified sewage flows
                clarified wastewater enters one end
                inoculated with aerobic bacteria (from sludge)
                vigorously aerated
                bacteria assimilate DOM and reproduce rapidly
                DOM thereby is converted to POM
                some is respired to CO2 and H2O
                   Pg (DOM) = Pn (POM) + R (CO2 + H2O)
                effluent is sent to secondary clarifier to remove sludge (POM)
                the sludge is "activated" sludge
                some of it is used to inoculate the incoming supernatant
                2° supernatant goes to tertiary treatment or post-treatment
                excess sludge is wasted back to the sludge treatment facility (more later)
                activated sludge can remove 90-95% of DOM

        c. Rotating Biological Contactors
               a new secondary treatment method
               is really specialized type of trickling filter
               1° supernatant flows over large rotating disks
               the disks increase surface area
               bacteria grow in thick layers on the disks

F. TERTIARY TREATMENT

      a. Objective
           removal of inorganic nutrients
           especially phosphorus, sometimes nitrogen

      b. Mechanisms
           chemical, physical, or biological
           no single, simple, inexpensive process
           usually expensive, unlike 1° and 2° treatment

      c. Uses
           most STPs do not have 3° treatment
           becoming more and more common as required by EPA and states


                                                                                                    11
               necessary to prevent eutrophication of receiving water

       d. Principles
             sometimes only the limiting nutrient need be removed
             usually phosphorus
             most, but not all, methods convert DIM to particulates, then settle

       e. Precipitation of Phosphorus
             2° supernatant treated to precipitate or coagulate P
             aluminum sulfate, aluminum chloride, ferric chloride, or an organic polymer
             precipitate or coagulant is settled, separated, sent to landfill

       f. Biological Nitrogen Removal
             relies on several species of nitrogen-metabolizing bacteria
             nitrogen, in various oxidation states, figures in the energy metabolism of many bacteria
             incorporated into special activated sludge tanks
             tank must be partitioned to maintain different oxygen regimes
             ammonifying bacteria mineralize organic nitrogen to ammonia under anoxic conditions
             C-NH2 è NH4
             chemoautotrophic nitrifying bacteria convert NH4 to NO3 under aerobic conditions
             NH4 è NO3
             nitrifying bacteria use NH4 as an energy source
             heterotrophic denitrifying bacteria use NO3 as an oxidizing agent to oxidize reduced
                carbon compounds for energy
             this reduces NO3 to N2
             NO3 è N2
             N2 released into atmosphere

       g. Biological Removal of Phosphorus
            phosphorus has no gaseous phase
            growing bacteria in activated sludge assimilate and store PO 4
            this converts soluble P to particulate P
            particulate P is removed by settling (2° clarifiers)

       h. Irrigation
             plants can be irrigated with treated wastewater
             St. Petersburg, FL irrigates 4000 acres of lawns, parks, golf courses with 2° effluent
             requires an expensive independent distribution system

       f. Wetland Ecosystems
            effluent can be routed through a constructed wetland for natural nutrient removal
            Orlando, FL uses a 1200 acre constructed wetland
            >1,000,000 marsh plants planted in a maze of shallow ponds and marshes
            creates an attractive wildlife habitat
            Phinzy Swamp Nature Park in Augusta is a constructed wetland


G. POSTTREATMENT
       a. Objectives
            always disinfect
            sometimes increase DO
            sometimes improve appearance
            sometimes filter

       b. Disinfection
            few pathogens survive 1° and 2° treatment


                                                                                                       12
                disinfection kills the few, if any, that remain
                required for public health reasons
                several methodologies available

           i. Chlorination
                  the most commonly used methodology
                  chlorine gas usually used
                  toxic and dangerous
                  sodium hypochlorite (bleach) can be used and is safer
                  residual chlorine kills organisms in receiving water
                  effluent must be dechlorinated
                  SO2 usually used for dechlorination

           ii. Ozone
                      *effective
                      *elevates DO
                      *but unstable and explosive
                      *must be generated on-site
                      *requires construction of an expensive ozone generating plant

           iii. Ultraviolet Irradiation
                    UV irradiation disinfects
                    has no effect on the receiving water
        c. Filtration
               a final filtration step sometimes added
               sand filters
               removes fine particulates and reduces BOD

        d. Aeration
             there may be a final aeration step
             reduces BOD and elevates DO

        e. Product
              clear, colorless, clean effluent with low BOD
              quality is often higher than that of receiving water
              if not for public resistance, this water could be potable
              it would be much easier and cheaper to convert this effluent to drinking water than to
                 start with raw water from a lake or river

H. SLUDGE TREATMENT
     recovered water goes into a receiving water
     but what can we do with the solids?

    1. Introduction
         a. Objective
              particulates have been removed at several steps in the process
              particulates collectively known as raw sludge
              it must be treated
              disinfected, BOD reduced, odor reduced, dried, disposed of

        b. Composition
             97-98% water
             black, reduced, foul-smelling, thick, potentially with pathogens


     2. Dewatering


                                                                                                        13
               raw sludge comes from 1° and 2° clarifiers
               it is dewatered in a sludge thickener (=sludge clarifier)
               particulates are allowed to settle
               water is decanted to influent (sent back to beginning)
               solids treated by various technologies

    3. Sludge Treatment Technologies
        a. Anaerobic Digestion
             an old, still used method
             dewatered sludge sent to large concrete anaerobic digestors
             anaerobic bacteria reduce the BOD
             convert organic solids to gasses (CH4, CO2, H2, and several other stinky gasses)
             much of the BOD is in these gasses
             this gas mixture is biogas and it is similar to natural gas (CH 4)
             biogas used to warm the digestors or burnt in the atmosphere (CO 2 and global warming)
             after 4-6 weeks treated sludge is stable, humus-like, and nutrient-rich, pathogens
                negligible
             it still has a high water content
             poured onto sand drying beds
             dried sludge can be used as soil supplement
             can be pressed into sludge cake for storage, transport, application using manure spreaders

       b. Land Application of Liquid Sludge
            dewatered, disinfected sludge can be applied directly to some crops or forests
            thickened sludge is disinfected using lime to raise pH to 12 for 2 hours
            Greenwood maintains a 350-acre hay farm for treated sludge disposal
            the hay is sold to local farmers
            life of the farm is limited by build-up of heavy metals in the soil

       c. Composting
            dewatered sludge is mixed with wood chips (timber industry waste)
            wood chips increase surface area exposed to oxygen
            aerobic bacteria break down the organic material
            sludge is converted to humus-like material and pathogens eliminated
            requires about 6-8 weeks
            wood chips are reused

       d. Co-Composting
            a similar process uses waste paper instead of wood chips
            the paper becomes part of the product
            no need to separate substrate from humus

       e. Pasteurization and Drying
             filter-pressed to dewater
             resulting sludgecake is dried in ovens to pasteurize it
             used for soil conditioner by farmers or homeowners
             may be formed into pellets which may be sold as soil conditioner

       *Note: municipalities never make a profit selling wastes
           *the principle motivation is to get rid of the product and, if possible, recoup some
               treatment costs
           *the motivation is usually to save space in the landfill



IV. SEPTIC TANKS


                                                                                                     14
       many homes are not connected to a sewage treatment facility
       they use individual septic tanks and drain fields
       resemble a combined clarifier and anaerobic digester
       object is to separate solids from liquid
       waste flows into underground tank
       solids settle to the bottom and are partly digested anaerobically
       liquid flows off the top into an underground drainfield
       liquid flows into the soil
       soil organisms reduce BOD
       overlying vegetation thrives
       solids must be pumped out periodically



Assignment:

   Questions:

   1.   Make a comment on the wastewater controversy regarding Tijuana River in Mexico.
                http://www.sdearthtimes.com/et0694/et0694s1.html

   2.   Make a report on using reclaimed wastewater for other purposes. Is there any hazard or danger for
        this?

   3.   Make comment on “Reusing Treated Wastewater in Domestic Housing: the Toronto Healthy
        House Project.”
                http://mha-net.org/msb/html/papers-n/palo01/wastewa.htm




                                                                                                      15

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:15
posted:12/9/2011
language:
pages:15