Wastewater Municipal Pretreatment Primary

EE304 ENVIRONMENT ENGINEERING & ABATEMENT PROCESS

EP317 ENVIRONMENT ENGINEERING AND ABATEMENT PROCESS

EG202 ENVIRONMENTAL SCIENCE AND TECHNOLOGY

Content

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EE304 ENVIRONMENT ENGINEERING & ABATEMENT PROCESS EP317 ENVIRONMENT ENGINEERING AND ABATEMENT PROCESS EG202 ENVIRONMENTAL SCIENCE AND TECHNOLOGY

WATER POLLUTION & WASTEWATER

0 Water quality parameter

0 Sources and classification of pollutants

0 Water quality control policy

0 Impact of pollutant to environment and

human

0 Waste water treatment technologies

Municipal Wastewater Treatment

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1) To protect water quality of the receiving watercourse through removal of:

Dissolved solids

Suspended solids

Pathogenic microorganisms

Offensive characteristics such as colour, foul odour

2) To reuse wastewater water reclamation


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Preliminary Treatment or Pretreatment

Primary Treatment

Secondary Treatment

Trickling Filters

Activated Sludge Processes

Rotating Biological Contactors

Stabilisation Lagoons

Anaerobic Processes

Tertiary or Advanced Treatment


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Pretreatment

Objectives: 1) To remove debris, grit and oily scums

2) To avoid shock loadings

3) To condition the wastewater for subsequent treatment processes


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Pretreatment

Consists of Bar Rack, Grit Chamber, Comminutor, Equalization Chamber.

Placed upstream of primary treatment to provide protection to the wastewaster treatment plant equipment.

Have little effect in reducing BOD.

In wastewater treatment plant with only soluble compounds, bar racks and grit chambers may be absent.


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Pretreatment 1) Bar Rack

Comes in pairs (two channels).

To remove large objects, e.g. rags, logs, paper, plastics, rocks, leaves.

Protect valves, pumps, other mechanical equipment.

Tracks are cleaned manually or mechanically.

Solid materials removed to sanitary landfill.


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Pretreatment 1) Bar Rack


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Pretreatment 2) Grit Chamber

To remove discrete particles. Grit inert dense material sand, broken glass, coffee

grounds, eggshells (about 1mm). Abrade pumps/ mechanical devices causing undue wear. Settle in corner and bends, reducing flow capacity, clogging

pipes and channels. Removal processes use gravity/velocity, aeration or

centrifugal force to separate the solids from the wastewater.

Based on: grit is heavier than the organic solids in suspension.


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Helical-flow aerated grit chamber

Consists of a cylindrical tank in which the flow enters tangentially, air introduced along one side of the channel near the bottom, creating a vortex flow pattern.

Grit settles by gravity into the bottom of the tank (grit hopper), while the lighter organic matter remains in suspension (effluent) and carried on to primary settling tank.


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Pretreatment 3) Comminutor

To chop up wastewater solids (rags, paper, plastics etc.) by revolving bars.

Reduce size of solids entering the plant;

avoid mechanical problem and clogging. Placed downstream of grit chambers to

protect cutting bars.


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Pretreatment 3) Comminutor


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Pretreatment 4) Flow Equalization Not a treatment process.

Used to dampen the variation in amount (flow) and strength of WW (BOD loading).

Can significantly improve the performance of existing plant and increase its useful capacity. In new plants, can reduce the size and cost (treatment units).

Usually achieved by constructing a large basin that collects the waterflow, and pump to the treatment plant at a constant rate.

Adequate aeration and mixing must be provided to prevent odors and deposition of solids.


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Pretreatment 4) Flow Equalization 1) In-line equalization:

All of the flow passes through the equalization basin. Can be used to achieve a considerable amount of constituent

concentration and flowrate damping. 2) Off-line equalization:

Only the flowrate above some pretreatment flow rate is deviated into the equalization basin.


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Pretreatment 4) Flow Equalization

In-line equalization Off-line

equalization

All the flow passes through the equalization basin.

Significant flow and load (BOD) dampening.

Only greater flow goes to the equalization basin.

Less effective dampening but pumping costs are much less than the in-line system.


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Pretreatment 4) Flow Equalization


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Primary Treatment

Objectives: 1) To remove readily settle-able solids & floating material,

thus reduce suspended solids content.

Light organic suspended solid remains in wastewater after pre-treatment;

Prepare wastewater for biological treatments;

Can be removed by (1) gravity in a sedimentation tanks (round or rectangular) or (2) by floating.


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Primary Treatment Sedimentation tanks (round or rectangular) also called

Primary Settling Tank / Primary Clarifier. 1) Raw sludge (mass of settled solid) removed by

mechanical scrapers and pumps. 2) Floating material (grease and oil) rise to the surface of

the tank- collected by a surface-skimming system. Sedimentation: process where the velocity of the

wastewater is lowered below suspension velocity & the suspended particles settled out due to gravity settling/clarification


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Primary Treatment Settling Tank

Primary Settling Tank


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50-60% suspended solid

and 30-35% BOD5 removal

capacities

Primary Settling Tank


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Design of Primary Settling Tank

1) Surface Loading Rate (SLR) or Surface Overflow Rate (SOR)

2) Hydraulic Detention/Retention Time (HRT)

3) Weir Loading


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Primary Treatment 1) SLR/SOR

Vo = QA

The surface overflow rate is a hydraulic loading factor expressed in terms of flow per surface area. This factor is also referred to as the surface settling rate or surface loading rate.

Vo = SOR, m3/m2.d

Q = average daily flow, m3/d A = total surface area of tank, m2


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Primary Treatment 1) SLR/SOR

A circular settling tank has a diameter of

120 ft. If the flow to the unit is 4.5 MGD,

what is the surface loading rate (in

gal/day/ft2)?

Question 1


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Primary Treatment 2) HRT

Hydraulic Detention/Retention Time (HRT) is the theoretical time wastewater is held in a clarifier based on the flow and volume of the tank.

t = HRT, hr Q = average daily flow, m3/d V = tank volume, m3

t = 24 V

Q


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Primary Treatment 2) HRT

900 m3/hr flow into a primary settling tank (L=54 m;

W=10 m; D=2.0 m).

(a) Calculate the retention time (t) in hours.

(b) Calculate the surface overflow rate.

Question 2


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Primary Treatment 3) Weir Loading

Weir loading rate is the calculation of how much water is being pushed into the launder weirs.

Launder weirs are installed at the basin or tank outlet to provide a uniform collection and flow-on of settled water.

Weirs are usually designed for a maximum flow rate based on optimum sedimentation conditions.


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Specified to prevent updraft that would carry solids out of the basin (solids washout).


Weir loading = Average Daily Flow

Weir Length

= Q

D


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A circular settling tank is 80 feet in diameter and has

a weir along its circumference. The effluent flow rate

is 2.75 MGD. What is the weir overflow rate in gallons

per day per foot?

Question 3



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Primary Treatment Floatation Floatation is used for the removal of suspended solid and oil

& grease from wastewater and for the separation and concentration of sludge.

The removal is achieved by injecting air bubbles into the water or wastewater in a flotation tank or basin. The small bubbles adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be skimmed off.

Polyelectrolyte addition can improve the adhesion of bubbles of flocs.


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Primary Treatment Floatation


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Primary Treatment

Wastewater Municipal Pretreatment Primary

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