Envi Chapter 5

University of San Carlos - Department of Chemical Engineering

University of San Carlos - Department of Chemical Engineering

Before 19th century in London, Paris and Boston: waste were excluded from the sewerage systems Prior to 19th century: city residents placed “night soil” in buckets along the streets and workers emptied the waste into “honeywagon” tanks - the waste was transported to rural areas for disposal over agricultural lands 19th century: invention of the flush toilet

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1842 in Hamburg, Germany: Lindley, English engineer, built the first “modern” sewerage system for wastewater carriage 1800s and 1900s: various treatment processes were tried 1920s: wastewater treatment had evolved to those processes in common use today

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Wastewater Characteristics Classification Industrial Wastewater - discharge to the municipal sewer if characteristics are compatible with municipal wastewater - require pretreatment to remove non-compatible substances prior to discharge into the municipal system - characteristics and treatment processes vary greatly from industry to industry

Municipal Wastewater - contains a wide variety of contaminants - constituents of wastewater may vary quantitatively - composition of wastewater may change slightly on a seasonal basis

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Table 1. Important wastewater contaminants Source

Environmental Significance

Domestic use, industrial waste, erosion by infiltration/inflow

Cause sludge deposits and anaerobic conditions in aquatic environment

Biodegradable organics

Domestic and industrial waste

Cause biological degradation, which may use up oxygen in receiving water and result in undesirable conditions

Pathogens

Domestic waste

Transmit communicable diseases

Nutrients

Domestic and industrial waste

May cause eutrophication

Refractory organics

Industrial waste

May cause taste and odor problems, may be toxic or carcinogenic

Heavy metals

Industrial waste, mining, etc.

Are toxic, may interfere with effluent reuse

Dissolved inorganic solids

Increases above level in water supply by domestic and/or industrial use

May interfere with effluent reuse

Contaminants Suspended solids

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Table 2. Typical analysis of municipal wastewater Constituent, mg/L Solids, total: Dissolved, total Fixed Volatile Suspended, total Fixed Volatile Settleable solids, mL/L Biochemical oxygen demand, 5-day, 200C (BOD5) Total organic carbon (TOC) Chemical oxygen demand (COD) Nitrogen (Total as N): Organic Free ammonia Nitrites Nitrates Phosphorus (total as P): Organic Inorganic Chlorides Alkalinity (ass CaCO3) Grease

Concentration Strong Medium Weak 1200 720 350 850 500 250 525 300 145 325 200 105 350 220 100 75 55 20 275 165 80 20 10 5 400 220 110 290 1000 85 35 50 0 0 15 5 10 100 200 150

160 500 40 15 25 0 0 8 3 5 50 100 100

80 250 20 8 12 0 0 4 1 3 30 50 50

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Terminology in Wastewater Treatment Unit operations – involve contaminant removal by physical forces Unit processes – involve biological and/or chemical reaction

Reactor – vessel, or containment structure, along with all of its appurtenances, in which the unit operation or unit process takes place

Wastewater – treatment system – composed of a combination of unit operations and unit processes designed to reduce certain constituent of wastewater to an acceptable level

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Table 3. Unit operations, unit processes and systems for wastewater treatment Unit operation, unit process or treatment system Contaminant Suspended solids

Sedimentation Screening and comminution Filtration variations Flotation Chemical-polymer addition Coagulation/sedimentation Land treatment system

Biodegradable organics

Activated – sludge variation Fixed – film: trickling filters Fixed – film: rotating biological contactors Lagoon and oxidation pond variation Intemittent san filtration Land treatment systems Physical – chemical systems

Pathogens

Chlorination Hydrochlorination Ozonation Land treatment systems

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Contaminant

Unit operation, unit process or treatment system

Nutrients: Nitrogen

Suspended – growth nitrification and denitrification variations Fixed – film nitrification and denitrification variation Ammonia stripping Ion exchange Breakpoint chlorination Land treatment systems

Phosphorus

Metal – salt addition Lime coagulation/sedimentation Biological – chemical phosphorus removal Land treatment systems

Refractory organics

Carbon adsorption Tertiary ozonation Land treatment system

Heavy metals

Chemical precipitation Ion exchange Land treatment systems

Dissolved inorganic solids

Ion exchange Reverse osmosis Electrodialysis

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Municipal wastewater – treatment systems Primary treatment – removal of solid materials from the incoming wastewater Secondary treatment – consists of biological conversion of dissolved and colloidal organics into biomass Tertiary treatment – involves further removal of suspended solids and/or the removal of nutrients

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Typical primary treatment system.

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Secondary treatment system (activated sludge system).

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Secondary treatment system (trickling filter system).

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Sludge treatment system.

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Primary Treatment Preliminary treatment – operations to eliminate large objects and grit, along with flow measurement

Screening - normally the first operation performed on the incoming wastewater

Screening Devices - used to remove coarse solids from wastewater - protect pumps and other mechanical equipment - prevent clogging of valves and other appurtenances

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Classification of Wastewater Screens FINE - consist of woven-wire cloth or perforated plates mounted on a rotating disk partially submerge in the flow or on a traveling belt - should be mechanically cleaned on a continual basis COARSE - consist of vertical bars spaced 1 or more centimeters apart - cleaned by manual raking for small plant and mechanically cleaned units for larger plants

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Things to Remember……… • Manually cleaned devices should be readily accessible for cleaning. • Mechanically cleaned systems should be enclosed in suitable housing. • Proper ventilation must be provided to prevent accumulation of explosive gases. • A straight channel section should be provided a few meters ahead of the screen. • Flow velocity should not exceed 1.0 m/s in the channel with 0.3 m/s considered good design. • Head loss across the screen will depend on the degree of clogging. • Clean bars and screens result in a head loss of less than 0.1 m.

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Comminuting Comminutor is located across the flow path and intercepts the coarse solids and shreds them to approximately 8mm in size. Barminutor uses a vertical bar screen with a cutting head that travels up and down the rack of bars, shredding the intercepted material.

Grit Removal Grit consists of inorganic solids such as pebbles, sand, silt, egg shells, glass, and metal fragments. Also contain larger heavier organics such as bone chips, seeds and coffee and tea grounds. Grit chambers are hydraulically designed to remove discrete particles with diameters of 0.2mm and specific gravity of 2.65.

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Table 4. Design parameter for aerated grit chambers. Value Item

Range

Typical

Dimensions: Depth, m

2–5

Length, m

7.5 – 20

Width, m

2.5- 7.0

Width-depth ratio Detention time at peak flow, min Air supply, m3/min m of length Grit and scum quantities: Grit, m3/103 m3

1:1–5:1

2:1

2–5

3

0.15 – 0.45

0.3

0.004 – 0.200

0.015

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Primary Sedimentation A unit operation designed to concentrate and remove suspended organic solids from the wastewater. Table 5. Design criteria for primary sedimentation tanks. Value Parameter Detention time, h

Range

Typical

1.5 – 2.5

2.0

Overflow rate, m3/m2 d Average flow

32 – 48

Peak flow

80 – 120

100

125 – 500

250

Depth

3 -5

3.6

Length

15 – 90

25 – 40

Width

3 – 24

6 – 10

0.6 – 1.2

1.0

3–5

4.5

3.6- 60

12 – 45

60 – 160

80

0.02 – 0.05

0.03

Weir loading, m3/m d Dimensions, m Rectangular

Sludge scraper speed, m/min Circular Depth Diameter Bottom slope, mm/m Sludge scraper speed, r/min

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Secondary Treatment Organic removal treatment and may consist of chemical-physical processes or biological processes.

Chemical-physical Operations Coagulation, microscreening, filtration, chemical oxidation, carbon adsorption, and other processes ca used to remove the solids and reduce the BOD to acceptable levels.

Biological Treatment Microorganisms use the organics in wastewater as a food supply and convert them into biological cells.

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Bacterial Growth in Pure Cultures

Characteristic growth curves of cultured micro-organisms (adapted from Monod, 1949).

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Kinetics of Bacterial Growth Biochemical equation for bacterial cell respiration organic matter + O2 + nutrients bacteria →CO2 + NH 3 + new biomass + other end products

For mixed culture

dX = µX dt where

dX = growth rate of biomass, mg/L/day dt X

= concentration of biomass, mg/L

µ

= specific growth rate constant, d -1

By integration

X = X 0 • exp( µt )

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Monod Equation

S µ = µm Ks + S where

S

= concentration of limiting substrate, mg/L

µm = maximum growth rate, d -1 Ks

= half-saturation constant

To take account of die-off

dX = µX − K d X dt Biomass production:

dX  µm S   X − K d X =  dt  K s + S 

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Rate of substrate utilization

where

Ideal:

dS dX − = dt dt

Real:

dS 1 dX − = dt Y dt

Y

= fraction of substrate converted to biomass, mg/L of biomass/mg/L of substrate = dX/dS

Substrate utilization:

dS 1  µm SX   − =  dt Y  K s + S 

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Secondary treatment systems are categorized as: ● Suspended growth ● Attached growth ● Dual biological suspended and attached growth Suspended growth systems Those aerobic processes that achieve a high micro-organism concentration through the recycle of biological solids. Attached growth systems Other known as fixed film reactors, allow a microbial layer to grow on the surface of the media while exposed to the atmosphere from where it draws its oxygen. Dual process systems Utilize two stage arrangements of fixed film and suspended growth processes with the objective of achieving very high quality effluent Standards.

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Activated Sludge Systems The more common activated sludge systems are: ● Complete mix ● Plug flow ● Oxidation ditch ● Contact stabilization ● Sequencing batch reactors

Complete Mix Reactors ♦ have uniform characteristics throughout the entire reactor ♦ tend to be circular or square ♦ aeration can be provided ♦ has the ability to withstand shock loads

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Typical complete mix activated sludge system.

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Mass balance of biomass production

Influent biomass + biomass production = effluent biomass + sludge wasted biomass

QoXo

dX V dt

(Qo − Qw) Xe

QwXw

 µ m SX  QoXo + V  − K d X  = ( Qo − Qw) Xe + QwXw  Ks + S  Mass balance of food substrate Influent substrate + substrate consumed = effluent substrate + sludge wasted substrate

QoSo

dS V dt

(Qo − Qw) Se

QwSw

 1  µ SX   QoSo − V   m   = ( Qo − Qw) Se + QwSw  Y  Ks + S  

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Plug Flow Reactors ♦ satisfactory mixing occurs in the lateral direction ♦ characterized by a high organic loading at the influent end of the basin ♦ has the ability to treat fully all influent and allow no ‘plugs’ to go untreated

Plug Influent

Effluent

Schematic of plug flow system.

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Typical plug flow activated sludge system.

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Oxidation ditch

Typical layout of an oxidation ditch system.

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Contact Stabilization ♦ a form of activated sludge where aeration is carried out in two phases ♦ aeration volume requirements are typically 50% lower than conventional plug flow ♦ used for expansion of existing systems and also in package plants

Layout of contact stabilization system.

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Sequencing batch reactors (SBRs)

Typical configuration for an SBR.

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