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E NVIRONMENTAL E NGINEERING 343 Lecture 9: Wastewater Philadelphia University Faculty of Engineering Department of Civil Engineering Second Semester, 2013/2014.

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Presentation on theme: "E NVIRONMENTAL E NGINEERING 343 Lecture 9: Wastewater Philadelphia University Faculty of Engineering Department of Civil Engineering Second Semester, 2013/2014."— Presentation transcript:

1 E NVIRONMENTAL E NGINEERING 343 Lecture 9: Wastewater Philadelphia University Faculty of Engineering Department of Civil Engineering Second Semester, 2013/2014

2 C OMPOSITION OF W ASTE WATER 99.9 % Water 0.1% Solids Solids 0.1% Inorganic (30%) Organic (70%) Suspended Organics (Colloidal – protein) Dissolved Organics (Carbohydrates) Dissolved In- organics (metals, salt, P,N) Suspended In-organics (Grit) Secondary Treatment Dissolved Organics and Suspended Organics Primary Treatment 60% SS; 30% BOD and Grit Tertiary Treatment

3 T YPICAL C OMPOSITION OF U NTREATED D OMESTIC W ASTEWATER [M ETCALF AND E DDY, 1991]. Concentration (mg/lt.) ContaminantsWeakMediumStrong Total Solids (TS) Total Dissolved Solid (TDS) Suspended Solid (SS) Settleable Solid51020 Biological Oxygen Demand (BOD 5 ) Total Organic Carbon (TOC) Chemical Oxygen Demand (COD) Nitrogen (total as N) Organic81535 Free ammonia Nitrites000 Nitrates000 Phosphorous (total as P)4815 Chlorides Sulfate Alkalinity (as CaCO 3 ) Grease Total coliform (no/100 ml)


5 Wastewater Treatment Plants Municipal treatment is divided into: Primary, Secondary and Tertiary Primary Treatment – removes solid materials from stream-Large debris may be removed by screens or reduced in size by grinding device. Inorganic solids are removed by the grit chamber Much of the organic suspended solids are removed by sedimentation Primary treatment removes 50% SS and 30% BOD treatment-waste

6 S CREENING - C HARACTERISTICS Screening is the first operation performed on wastewater. The purpose of screening is to remove coarse solids e.g. sticks, rags, boards, napkins etc. which can clog valves and pipes and damage pumps. Screens -Placed in rectangular channel -Properly ventilated to prevent accumulation of gases -A straight channel ahead of the screens to provide distribution of flow -Flow velocity 0.3m/s and not greater than 1.0m/s Coarse -Vertical bars >1cm - Inclines away from flow -Cleaned manually by raking for small plants -Mechanically cleaned in large plants Fine -Woven wire cloth -Or perforated plates mounted on a rotating disk or drum or on a travelling belt -Mechanically cleaned on a continuous basis Screening

7 Screened solids are caught with raw sewage: - Promptly disposed in sanitary landfill - Grind and returned to wastewater flow - Incineration Screening


9 -L ocated across the flow path and intercepts the coarse solids and shred them to approximately 8mm in size. -Comprise of a screen and cutting teeth -High maintenance equipment and provisions should be made to by-pass flow during repairs. Small plants use screens as by-pass while large plants install comminutors in parallel. Comminutor hotolibrary_Wastewater_Images.shtm

10 Grit Chamber The objective of the Grit Chamber is to remove inorganics, and some larger organics e.g. pebbles, sand, silt, egg shells, glass, metal fragments, bone chips, seeds, coffee and tea grounds. Why we remove Grits? -Grits are abrasive in nature and will cause wear on pumps -Grit deposits in pipes, sumps and clarifiers can absorb grease and solidify. -They are non-biodegradable and occupy valuable space in the digester What are the sources of Grits- -The major contributor is infiltration and depends on the type, age and condition of sewerage systems -Industrial waste -Domestic garbage grinders

11 Flow measurement does not remove contaminants, however, knowledge of hydraulic loading rates is critical to the operations of the reactors. Hydraulic loading influences operation parameters as air flow, recirculation rates, infiltration rates, inflow quantities Flow Measurement Methods of Flow Measurements a. Digital Flow meters b. Parshall Flume

12 How does the system work? Primary sedimentation is a unit operation designed to concentrate and remove suspended organic solids from wastewater. Most of the SS in wastewater are “sticky” in nature and flocculate naturally. Primary settling proceeds as type-2 settling without the addition of chemical coagulation, mechanical mixing and flocculation operation. Organic matter is slightly heavier than water and settles slowly 1.0 to 2.5m/h. Oil and grease float to the surface and must be skimmed off. Scum is skimmed off and sent to the scum tank and disposed with screen material, grit or digested sludge Sludge should be removed from the primary sedimentary tank before anaerobic condition develop. Hydraulic detention time in sedimentation basin range from hrs under average flow conditions. A 2.0 hr detection time is typical Primary Sedimentation Tank


14 What are the Objectives? Effluent from primary treatment still contains 40-50% original suspended solids and virtually ALL dissolved organics and inorganics The removal of organics both suspended (colloidal) and dissolved is called secondary treatment. How are these objectives achieved? The use of biological processes is the most cost effective method used to treat municipal wastewaters. Microorganisms use organics in wastewater as a food source and convert them to biological cells (Biomass) Because wastewater contains a wide variety of organics, a variety of organisms or mixed culture is required for complete treatment. Secondary Treatment Processes

15 Efficient treatment then depends on understanding the requirements for optimal growth as well as recognizing unfavorable conditions. Typically, a domestic wastewater prior to entering the treatment plant will contain from 100,000 to 1,000,000 microorganisms per milliliter. While all microorganisms found in wastewater treatment plants have some role in the decomposition of wastes, probably the three most significant microbial groups are the bacteria, fungi, and protozoa. Bacteria have the primary role of decomposing wastewater compounds, forming settleable solids, and at times are the source of operational problems. The general group called fungi are significant since many operational problems are caused by members of this group. Protozoa are microorganisms that play a key role as predators and help control the bacterial populations. Microorganisms in Wastewater

16 Activated Sludge System A biological treatment process in which a mixture of sewage and activated sludge is completely mixed and aerated (sufficiently). In this process the microorganisms are mixed thoroughly with the organic compounds under conditions that stimulate their growth through us of organic compounds as foods. As the microorganisms grow and are mixed by the agitation of the air, the individual organisms clumps together (flocculate) to form an active mass of microbes (biological flocs) called activated sludge Air is continuously injected to mix the activated sludge to supply the oxygen needed for the M.O to break down the organic matter.

17 Activated Sludge System

18 The activated sludge (Biological Solids) is subsequently separated from the treated sewage by settlement and 22 to 30 % is re-used (called return sludge) to maintain high population of microbes to permit breakdown of organic matter. The remaining sludge is discarded (called waste activated sludge) The activated sludge is controlled by balancing between waste activated sludge and return activated sludge. If too low, less effective If too high, overflow the secondary tank and flow into the receiving streams Sludge age/solids retention time (SRT) the average amount of time that microorganisms are kept in the system; measured in days In conventional activated sludge systems, the wastewater is aerated for 6-8 hrs ; in long, rectangular aeration tanks Activated Sludge System –

19 W ASTEWATER T REATMENT P LANTS Sludge Digestion Sludge from primary treatment as well as the secondary clarifier are sent to the sludge digestion unit where it is undergoes anaerobic degradation. The results are: Methane – Used for power generation CO2 Liquid – contains high organic – recycled through the plant Inert solids – contains high minerals – soil conditioner/ fertilizer on agricultural lands


21 Trickling Filter consist of a bed of coarse material, such as (stone or plastic materials) over which wastewater is applied Stones are used to increase the surface area for bio- film growth (microorganisms) that biodegrade the waste. Under high organic load, the void space between the rocks plug causing flooding, and failure of the system restriction of the amount of oxygen available for microbes Odour and filter flies Part of the liquid effluent may be recycled to the trickling filter for additional treatment or to maintain hydraulic flow rates S ECONDARY W ASTEWATER T REATMENT P LANTS



24 Oxidation ponds / Waste stabilization pond / sewage logons Self purification Aerobic Facultative Anaerobic Aerated Disinfection: Mainly by chlorine W ASTEWATER T REATMENT P LANTS

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