e.eng lect 13

Wastewater Treatment Methods Physical Unit Operations Chemical Unit Operations Biological Unit Operations

Before discussing the wastewater treatment methods let us first have a brief look on some characteristics of waste water,

Waste-water quality may be defined by its physical, chemical, and biological characteristics.

Removal

Treatment

Disposal

Physical parameters include color, odor, temperature, and turbidity. Insoluble contents such as solids, oil and grease, also fall into this category. Solids may be further subdivided into suspended and dissolved solids as well as organic (volatile) and inorganic (fixed) fractions.

Chemical parameters associated with the organic content of waste-water include biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), and total oxygen demand (TOD).

Inorganic chemical parameters include salinity, hardness, pH, acidity and alkalinity, as well as concentrations of ionized metals such as iron and manganese, and anionic entities such as chlorides, sulfates, sulfides, nitrates and phosphates.

Bacteriological parameters include coliforms, fecal coliforms, specific pathogens, and viruses. Both constituents and concentrations vary with time and local conditions. Waste-water is classified as strong, medium or weak, depending on its contaminant concentration.

Physical, chemical and biological methods are used to remove contaminants from waste-water. In order to achieve different levels of contaminant removal, individual waste-water treatment procedures are combined into a variety of systems, classified as primary, secondary, and tertiary waste-water treatment.

Among the first treatment methods used were physical unit operations, in which physical forces are applied to remove contaminants. Today, they still form the basis of most process flow systems for wastewater treatment. This section briefly discusses the most commonly used physical unit operations.

The screening of waste-water, one of the oldest treatment methods, removes pollutants from the waste stream to protect downstream equipment from damage, avoid interference with plant operations and prevent objectionable floating material from entering the primary settling tanks.

Screening devices may consist of parallel bars, rods or wires, grating, wire mesh, or perforated plates, to intercept large floating or suspended material. The openings may be of any shape, but are generally circular or rectangular. The material retained from the manual or mechanical cleaning of bar racks and screens is referred to as “screenings”, and is either disposed of by burial or incineration.

Comminutors are used to pulverize large floating material in the waste flow. They are installed where the handling of screenings would be impractical.

Their use reduces odors, flies and unsightliness. A comminutor may have either rotating or oscillating cutters. Rotating-cutter comminutors either engage a separate stationary screen alongside the cutters, or a combined screen and cutter rotating together.

Sedimentation, a fundamental and widely used unit operation in waste-water treatment, involves the gravitational settling of heavy particles suspended in a mixture. This process is used for the removal of grit, particulate matter in the primary settling basin.

Sedimentation takes place in a settling tank, also referred to as a clarifier. There are three main designs, namely, horizontal flow, solids contact and inclined surface.

Flotation is a unit operation used to remove solid or liquid particles from a liquid phase by introducing a fine gas, usually air bubbles. The gas bubbles either adhere to the liquid or are trapped in the particle structure of the suspended solids, raising the buoyant force of the combined particle and gas bubbles.

Particles that have a higher density than the liquid can thus be made to rise. In waste-water treatment, flotation is used mainly to remove suspended matter and to concentrate biological sludge. The chief advantage of flotation over sedimentation is that very small or light particles can be removed more completely and in a shorter time. Once the particles have been floated to the surface, they can be skimmed out.

Dissolved-air flotation Air flotation Vacuum flotation Chemical additives

The filtration of effluents from waste-water treatment processes is a relatively recent practice, but has come to be widely used for the supplemental removal of suspended solids from waste-water effluents of biological and chemical treatment processes, in addition to the removal of chemically precipitated phosphorus. The complete filtration operation comprises two phases: filtration and cleaning or backwashing.

The waste-water to be filtered is passed through a filter bed consisting of granular material (sand, anthracite and/or garnet), with or without added chemicals. Within the filter bed, suspended solids contained in the waste-water are removed.

The phenomena that occur during the filtration phase are basically the same for all types of filters used for waste-water filtration. The cleaning/backwashing phase differs, depending on whether the filter operation is continuous or semi continuous. In semi-continuous filtration, the filtering and cleaning operations occur sequentially, whereas in continuous filtration the filtering and cleaning operations occur simultaneously.

There are two types of sand filtration processes:

rapid gravity sand filtration, and slow sand filtration.

Chemical processes used in waste-water treatment are designed to bring about some form of change by means of chemical reactions. They are always used in conjunction with physical unit operations and biological processes.

This section discusses the main chemical unit processes, including chemical precipitation, adsorption, disinfection, dechlorination and other applications.

“Process of removing fine size suspended particles by addition of necessary amount of suspended chemicals called co-agulants, to water before sedimentation”.

Common co-agulants in use are: Alum K2SO4.Al2(SO4)3.24H2O Ferrous Sulphate FeSO4·7H2O

• Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 +3CaSO4 + 6CO2

co-agulant calcium bicarbonate gelatinuous floc

• FeSO4 + Mg(HCO3)2 Fe(OH)2 +MgCO3 + CO2 + H2O

• 4Fe(OH)2 +2H2O + O2 4Fe(OH)3

Dissolved oxygen Floc

o Coagulants are generally added in solution & for their proper mixing mixers are employed

Flocculation

Adsorption is the process of collecting soluble substances within a solution on a suitable interface.

Activated carbon is produced by heating char to a high temperature and then activating it by exposure to an oxidizing gas at high temperature. The gas develops a porous structure in the char and thus creates a large internal surface area. A fixed-bed column is often used to bring the waste-water into contact with AC. The water is applied to the top of the column and withdrawn from the bottom, while the carbon is held in place.

Disinfectants act through one or more of a number of mechanisms, including damaging the cell wall, altering the colloidal nature of the protoplasm and inhibiting enzyme activity.

In applying disinfecting agents, several factors need to be considered: contact time, concentration and type of chemical agent, intensity and nature of physical agent, temperature, number of organisms, and nature of suspending liquid.

The process of destroying disease causing bacteria, microorganisms etc. from water is known as disinfection of water.

• Sterilization by Bleaching powder

• Sterilization by Chlorine

• Sterilization by chloramine

• Sterilization by ozone

• Sterilization by ultravilet light

Limitation: Only limited quantity must be used… Drawbacks: Introduces calcium in water Always analyzed before use

baffles

Raw water Chlorine

Sterilized water

microbial inactivation

Commonly used means of disinfection include the following:

Physical agents such as heat and light. Mechanical means such as screening,

sedimentation, filtration, and so on. Radiation Chemical agents including chlorine and its

compounds, bromine, iodine, ozone, phenol and phenolic compounds, alcohols, heavy metals, dyes, soaps and synthetic detergents,

Dechlorination is the removal of free and total combined chlorine residue from chlorinated wastewater effluent before its reuse or discharge to receiving waters. Chlorine compounds react with many organic compounds in the effluent to produce undesired toxic compounds that cause long-term adverse impacts on the water environment and potentially toxic effects on aquatic micro-organisms.

In these processes, micro-organisms, particularly bacteria, convert the colloidal and dissolved carbonaceous organic matter into various gases and into cell tissue which is then removed in sedimentation tanks.

Activated-sludge process Aerated lagoons Trickling filters Rotating Biological contactors

Sanitary sewers Bar screens Primary settling basins Aeration basins Final settling basins Primary sludge Digesters Subsurface injection

They carry wastewater from homes and businesses to the raw wastewater pumping station at the treatment plant. The wastewater flows by gravity, rather than pressurized pipe flow, in the sanitary sewer pipes. Routine cleaning and closed circuit television inspection of sanitary sewer lines helps keep the sewer collection system in good shape.

They let water pass, but not trash (such as rags, diapers, etc.). There are two bar screens located inside the Raw Wastewater Pump Building. The trash is collected and properly disposed off. The screened wastewater is pumped to the Primary Settling Basins.

They allow smaller particles to settle from wastewater by gravity. This primary wastewater flows out to the next stage of treatment. Scrapers collect the solid matter that remains (called "primary sludge"). A surface skimmer collects scum or grease floating on top of the basins.

They supply large amounts of air to the mixture of primary wastewater and helpful bacteria and the other microorganisms that consume the harmful organic matter. The growth of the helpful microorganisms is speeded up by vigorous mixing of air (aeration) with the concentrated microorganisms (activated sludge) and the wastewater. Adequate oxygen is supplied to support the biological process at a very active level. The ratio of food (organic matter) to organisms to oxygen is continually monitored and adjusted to meet daily variations in the wastewater.

They allow the clumps of biological mass (the microorganisms) to settle from the water by gravity. 90-95 % of this mixture, called "activated sludge," is returned to the aeration basins to help maintain the needed amount of microorganisms. The remaining 5-10 % is pumped to the anaerobic digester.

The "primary sludge" from the Primary Settling Basins is pumped to the Hydrocyclone Grit Separator where it is spun, thereby separating the inorganic solids (grit) from the lighter weight organic solids. The grit is disposed of in the city landfill.

The primary sludge continues on to the gravity Sludge Thickener where the solids are concentrated and pumped to the anaerobic digesters. The liquid overflow is returned to the pump station. Waste Activated Sludge from the Final Settling Basins is pumped to a Centrifuge for further solids processing, then pumped to the anaerobic digestor.

Primary and activated sludges are anaerobically digested (decomposed by bacteria without the presence of air) in the two-stage digesters. Stabilized sludge has little odor and conforms to the EPA requirements to further reduce harmful microorganisms

Methane gas is produced by this anaerobic digestion and is used as fuel for an engine-generator providing 240 kW of electrical power used in the treatment process. Waste heat from the engine is recovered for heating the treatment plant buildings and to provide heating to improve the sludge digestion process and produce more gas.

Aplication of the stabilized sludge (biosolids) onto both City-owned and private farmland by subsurface injection (plowing). The biosolids are utilized in an environmentally acceptable manner as a beneficial and valuable fertilizer and soil conditioner. The biosolids applied to all sites are monitored for nutrients, metals, other compounds and fecal coliform bacteria. Soil testing is performed at all sites prior to biosolids application.