Sewage

Question 1

What percentage of the UK is served by large & small scale treatment works?

Answer 1

96%

Question 2

What percentage of the UK are served by septic tanks and cesspools?

Answer 2

4%

Question 3

What are the 3 key components to any sewerage treatment technique?

Answer 3

1. Screening
2. Sedimentation
3. Bacterial degradation

Question 4

Give 6 examples of what makes up sewage?

Answer 4

Domestic wastewater
Mineral solids
Dissolved pollutants
Faecal matter
Paper
Urine
Sanitary items
Food residues
Living organisms

Question 5

What is a cesspool/cesspit?

Answer 5

A storage facility for sewage prior to its removal by honey sucker for treatment elsewhere.

Question 6

Key facts regarding cesspools?

Answer 6

Cheapest method where not served by sewers.
Periodic emptying required.
Installation is discouraged but may be only option in some rural areas.
MUST NOT have an outlet. (Environmental Pollution)
Minimum size 18m³ (18,000 litres).
Only foul water permitted to enter, surface water directed elsewhere.
Must be water tight, and vented.
Removed by LA or specialist contractor periodically, using ‘Honeysucker’

Question 7

Where would you see Septic tanks, what are the 3 treatment stages and up to how many people can they operate effectively.

Answer 7

Used where sewerage installation is impractical – remote areas

Consist of up to 3 treatment stages:
Primary settlement tank
Biological filter
Humus tank

Serve populations <300

Question 8

What are the two large scale sewerage management processes called?

Answer 8

Traditional
Activated Sludge

Question 9

What are the 3 sewerage network influent loadings?

Answer 9

Domestic wastewater known as “foul water”
Low grade industrial discharges known as “trade effluents”
Run off from roads, land, roofs, etc known as “storm water”

Question 10

What are the 6 stages in the sewage treatment process and up to what treatment as a minimum standard is the legislation requirement? finally, why would you further process sewage?

Answer 10

Preliminary Treatment
Primary Treatment
Secondary Treatment
Tertiary Treatment
Advanced Treatment
Final Discharge

Legislation requires at least, up to and including secondary treatment as a minimum standard

Number of stages depends on sensitivity of watercourse receiving the final effluent discharge.

Question 11

In the UK a sewage treatment works (STW) are issued with a discharge consent in order to limit pollution of watercourses receiving treated effluent. This consent includes a flow limit based on Dry Weather Flow what is this?

Answer 11

DWF is currently defined as the average effluent flow during a 7-day period of dry weather.

Question 12

What influences the quantity of sewage received at the works?

Answer 12

Time of day – related to water usage high in the morning etc bathing, shaving ect.

Time of year – seasonal irrigation trends

Direct and indirect rainfall.

Question 13

What is the maximum operating capacity of STWs?

Answer 13

3 times the DWF rate.

Question 14

What happens if the sewage received by the STW is over the operating capacity?

Answer 14

flows between 3 & 6 x DWF is diverted into separate storm tanks for temporary storage.
In the rare event of 6 x DWF being exceeded the surplus is discharged direct to the watercourse as dilution rates are deemed as sufficiently high.

Question 15

What are the 4 components of preliminary treatment?

Answer 15

Coarse screening

Comminutor/shredder

Grit chamber

Dewaterer

Diversion to storm water holding tanks may be included at this point.

Question 16

What is the purpose of the comminutor?

Answer 16

Purpose is to break up large material to increase its surface area for sedimentation and subsequent biological action.

Question 17

What is the purpose of the grit chamber?

Answer 17

To prevent larger abrasive particles from entering the process.

Question 18

What occurs in the de-waterer and what is done with the by product?

Answer 18

Macerated screenings are pumped onto first mesh where excess water is removed.

Moves on to second mesh where residual water is squeezed out by a hydraulic ram prior to disposal by:

Landfill/burial (usually)
Incineration.

Question 19

What occurs during the primary treatment?

Answer 19

A sedimentation phase in the process in which 2 waste streams are produced:

Sewage travels at low velocity - fine particles settle out forming a sludge which is removed to either a drying bed or a digester.

primary tank effluent or ‘settled sewage’ flows to the next stage.

Question 20

What is the purpose of the Primary treatment?

Answer 20

Removes heavier solid particles (50% to 80% of the suspended material)

Removes some organic pollution (30-50% BOD)

Removes c10% of total bacteria

Retains grease and scum

Helps reduce loading on downstream biological processes.

Question 21

What is the aim of the secondary treatment?

Answer 21

The aim of the biological treatment stage is oxidation of the settled sewage by removing dissolved organic material

Question 22

In traditional sewage plants the biological filtration method is used, what is the role of this process? and 2 organisms that assist?

Answer 22

Process relies on aerobic bacteria and micro-organisms to break down the impurities in the settled sewage.

The micro-organisms comprise mainly:
Nitrogen reducing bacteria
Psychoda fly larvae
Worms
Algae.

Question 23

Sometimes called final clarification tanks and Similar in design to primary settlement tanks, what is the role of the final or humus settlement process?

Answer 23

Purpose to remove the smaller solids from the filter effluent

And to ensure a good quality effluent which can be discharged to a watercourse.

Question 24

Give 4 examples of tertiary treatments?

Answer 24

Fish lagoons
Micro straining
Sand filtration
Reed beds
Grass beds
Clarifiers
Drum Filters
Nitrifying Filters.