ReflectionsWhat are the two broad tasks of environmental engineers?What is the connection between the broad tasks of environmental engineers and building a water treatment plant?Why may the water need to be changed/treated?

Simple SortingGoal: clean waterSource: (contaminated) surface waterSolution: separate contaminants from waterHow?

Where are we going?particlesdissolved chemicalspathogensUnit processes* designed to remove ___________remove __________ ___________inactivate __________*Unit process: a process that is used in similar ways in many different applicationssedimentationfiltration...

Unit Processes Designed to Remove Particulate MatterScreeningSedimentationFiltrationDisinfectionStorageDistribution

Conventional Surface Water TreatmentScreeningSedimentationFiltrationDisinfectionStorageDistributionRaw watersludgesludgesludge

ScreeningRemoves large solidslogsbranchesragsfishSimple processmay incorporate a mechanized trash removal systemProtects pumps and pipes

Sedimentationthe oldest form of water treatmentuses gravity to separate particles from water

Filtrationis commonly the mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass.The fluid that passes through is called the filtrate.

DisinfectionDisinfection: operations aimed at killing or ____________ pathogenic microorganismsIdeal disinfectant_______________ _____________________________________________ _______________inactivatingToxic to pathogensNot toxic to humans Fast rate of kill Residual protectionEconomical

Disinfection OptionsChlorinechlorine gassodium hypochlorite (bleach)Irradiation with Ultraviolet lightSonicationElectric Current/ElectrocoagulationPoisonous gas risk of a leak

Removal of Dissolved Substances (1)Aeration (before filtration)oxidizes iron or manganese in groundwateroxidized forms are less soluble and thus precipitate out of solutionremoves hydrogen sulfide (H2S)Softening (before filtration)used to remove Ca+2 and Mg+2usually not necessary with surface waters

Removal of Dissolved Substances (2)Activated Carbon (between filtration and disinfection)extremely adsorbentused to remove organic contaminantsReverse Osmosissemi-permeable membrane allows water molecules to pass, but not the larger ions and moleculesprimarily used for desalinationalso removes organic materials, bacteria, viruses, and protozoa

ClearwellBackwash Lagoon

PROCESS

ScreeningScreening is the first operation at any wastewater treatment works. This process essentially involves the removal of large non-biodegradable and floating solids that frequently enter a wastewater works, such as rags, papers, plastics, tins, containers and wood.Efficient removal of these constituents will protect the downstream plant and equipment from any possible damage, unnecessary wear & tear, pipe blockages and the accumulation of unwanted material that will interfere with the required wastewater treatment processes. Wastewater screening is generally classified into either coarse screening or fine screening. Screens may be manually or mechanically cleaned, with only the older and smaller treatment facilities using manually cleaned screens as their primary or only screening device. Coarse screens are typically used as primary protection devices, and usually have openings of 10mm or larger.

Screener

Grit ChamberGrit chambers are basin to remove the inorganic particles to prevent damage to the pumps, and to prevent their accumulation in sludge digestors.

What problems can grit cause in a wastewater treatment system?At any point in a wastewater collection or treatment system where flow velocity is less than 1.5 ft/sec, grit will begin to fall out of suspension and collect. These low velocity areas can include drains, channels and pipelines. As grit collects in unintended areas, clogs can form, stopping wastewater flow. The trapped organic matter in these areas can then rapidly form pockets of anaerobic (i.e., septic) digestion that produce gaseous by-products that are harmful to both collection and treatment system surfaces as well as human health.

Grit is extremely abrasive and will produce abnormal wear on pumps and treatment system surfaces, significantly reducing the operational life of a wastewater treatment system and its components. The life of a pump impeller can be reduced by as much as 30 percent due to the abrasive action of grit.

What problems can grit cause in a wastewater treatment system?

What problems can grit cause in a wastewater treatment system?Grit will accumulate within the major treatment basins (e.g., waste stabilization ponds), eventually causing a loss in treatment capacity. In addition, since grit quickly settles out of wastewater and to the bottom of basins, the cumulative effect of the grit is often not known until major problems arise in loss of treatment efficiency

How does grit chamber operates?Grit chambers are nothing but like sedimentation tanks, designed to separate the intended heavier inorganic materials (specific gravity about 2.65) and to pass forward the lighter organic materials. Hence, the flow velocity should neither be too low as to cause the settling of lighter organic matter, nor should it be too high as not to cause the settlement of the silt and grit present in the sewage. This velocity is called "differential sedimentation and differential scouring velocity". The scouring velocity determines the optimumflow through velocity.

CLARIFIER

A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and (or) thickening. Concentrated impurities, discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum. Some of the solids collected in the secondary clarifier (return activated sludge) are sent back to the aeration tank to treat more wastewater and the excess (waste activated sludge) is pumped to another location in the plant for further treatment. The clean water that flows out the top of the clarifier is sent along for disinfection.

Clarifier operationIn order to maintain and promote the proper processing of a clarifier, it is important to remove any corrosive, reactive and polymerisable component first, or any material that may foul the outlet stream of water to avoid any unwanted side reactions, changes in the product or cause damage to any of the water treatment equipment. This is done by routine inspections and the frequent cleaning of the quiescent zones and the inlet and outlet areas of the clarifier in order to ascertain the extent of sediment build up and to clean and remove any scouring, litter, weeds or debris that may have accumulated over time.[14]

Clarifier operationWater being introduced into the clarifier should be controlled to reduce the velocity of the inlet flow. Reducing the velocity maximizes the hydraulic retention time inside the clarifier for sedimentation and helps to avoid excessive turbulence and mixing; thereby promoting the effective settling of the suspended particles. To further discourage the overt mixing within the clarifier and increase the retention time allowed for the particles to settle, the inlet flow should also be distributed evenly across the entire cross section of the settling zone inside the clarifier, where the volume is maintained at 37.7 percent capacity.The sludge formed from the settled particles at the bottom of each clarifier, if left for an extended period of time, may become gluey and viscous, causing difficulties in its removal. This formation of sludge, promotes anaerobic conditions and a healthy environment for the growth of bacteria. This can cause the resuspension of particles by gases and the release of dissolved nutrients throughout the water fluid, reducing the effectiveness of the clarifier. Major health issues and problems can also occur further down the track of the water purification system or hinder the health of the fish found downstream of the clarifier.

Aeration

Aeration is used in water treatment as a pretreatment in the process of removing iron and hydrogen sulfide (rotten egg smell) from water.

It is a process of exposing water to air by dividing the water into small drops, by forcing air through the water, or by a combination of both. Aeration is used to add oxygen to water and to remove carbon dioxide, hydrogen sulfide, and taste-producing gases or vapors.

Air is a powerful oxidizer of both iron and hydrogen sulfide. It quickly converts unfilterable ferrous iron to filterable ferric iron, and it reduces hydrogen sulfide to elemental sulphur, which is easily removed from water by a filter. Aeration is also an effective method of bacteria control.

METHODS OF AERATIONThe water-fall aerator. Through the use of spray nozzles, the water is broken up into small droplets or a thin film to enhance countercurrent air contact.

In the air diffusion method of aeration, air is diffused into a receiving vessel containing counter-current flowing water, creating very small air bubbles. This ensures good air-water contact for "scrubbing" of undesirable gases from the water.