Low Pressure Sewers

Question 1

What are the main elements of a LPS?

Answer 1

o Pumping station with grinder
o Small diameter pipes in polyethylene
o If above frost depth: pipe insulation and heating cable
o Other elements: check-valves, air release valves, H2S treatment units

Question 2

Which types of grinder pumps are found?

Answer 2

o Screw pump
 Narrow flow range
 Broad pressure range
o Centrifugal pumps
 Broad flow range
 Narrow pressure range

Question 3

Describe the share of responsibility of LPS

Answer 3

o Property owner:
 Installs the pumping station (including electrical connection)
 provides and pay the electricity to operate the pumping station
 contacts the water utility if the alarm of the pumping station is on.
o Water utility:
 sizes, purchases, maintains and repairs the pumping station.
o Requires a bit more coordination than a gravity sewer connection, responsibility if something doesn’t work? Wrong installation or malfunctioning of the pumping station?

Question 4

What are the pros with gravity sewers?

Answer 4

• No moving parts, no need for electricity.
• One pumping station per catchment.
• Clear split of responsibility with property owner and water utility.
• Wastewater has access to oxygen: formation of H2S gas is limited.
• Pipes can be inspected.

Question 5

What are the pros with LPS?

Answer 5

• No need for negative pipe slope + smaller pipe diameter  less excavation needs lower cost/m
• Usually shorter pipe length
• Can be suitable in areas with: hills, shallow bedrock, shallow groundwater, trees that should be saved, existing houses and roads.
• Very low risk for infiltration/inflow and blockages.

Question 6

What are cons with gravity sewers?

Answer 6

• In flat areas: need to lay pipes “deeper and deeper” to the pumping station, or built more pumping stations..
• Need public space to build pumping station
• Risk for sewer blockages if self-cleansing velocities aren’t met (change of flow conditions in the future, change of slope due to ground movements) or if wrong objects are flushed in the toilets
• Infiltration and inflow problems (very limited in new systems though)
• Risk for basement floodings

Question 7

What are the cons with LPS?

Answer 7

• Many pumping stations to maintain (one per house)
• More dependence to the electricity network
• Tenants needs to keep an eye on the alarm system
• More interaction needed between water utility and property owner during connection process.
• Water utility needs to access the pumping station situated on a private property.
• No access to oxygen in the pipe network leads to H2S emissions with risk for odour and corrosion, if not properly handled during design.
• Higher risk for wastewater exfiltration

Question 8

What is the minimum velocity of a LPS?

Answer 8

0.7m/s once a day

Question 9

What assumptions are made with the rational method?

Answer 9

 Residential only, assuming single house lots with one pump per dwelling.
 With consistent loading rates and diurnal patterns across the catchment.
 Catchments with pumps giving consistent head – flow curves of a near vertical nature

Question 10

Describe the rational method

Answer 10

An empirical relationship between the number of accumulated pumps in a segment of pipe, and the statistical number of pumps expected to run simultaneously to give the maximum flow.

Question 11

Describe the design procedure of LPS

Answer 11

o Identify or create zones
o Identify max pump rate – usually from pump manufacturer based on pipe size or design tables