HEALTH AND SANITATION
19
Rainwater harvesting:
overflow design and
maintenance
To prevent contaminants from entering into the tank, the
overflow drainage piping design should be similar to the
conveyance network - that is, it should be structurally sound with
no points of entry other than those required for water flow.
By Water Research Commission
If overflow handling provisions are not in
place, excess rainwater will back up rainwater
conveyance and top-up drainage piping, until it
reaches a point from which it can most easily
discharge/overflow.
www.watercache.com
When connecting to the storm sewer, either an air
break, or a special type of check valve, called a
“backwater valve” must be installed on the overflow
drainage piping. During intense rainfall events,
rainwater can potentially back up in the storm
sewer, causing contaminated water to back up
into the rainwater storage tank. Backwater valves
act like backflow preventers, preventing the water
from the storm sewer from entering the rainwater
storage tank.
Design and installation of overflow
1. Determine the overflow discharge location and
method
a) Overflow discharge locations include: grade,
storm sewer or soakaway pit.
b) Overflow discharge methods include: gravity flow
or pump-assisted flow.
c) Overflow by pump-assisted flow is not
recommended.
d) Consult the applicable National/Provincial/Local
codes and regulations, municipal by-laws and
local authorities regarding the permitted overflow
discharge locations.
e) Evaluate the feasibility of the overflow discharge
locations.
i. Overflow to grade
• The overflow discharge location must be at a
lower elevation than the flood level rim of the
tank for gravity flow to be feasible.
ii. Overflow to storm sewer
• A storm sewer connection must be present
at the site.
August 2019 Volume 25 I Number 6
An example of a combined system with above and below ground tanks and water
treatment system.
• The overflow discharge location must be at a
lower elevation than the flood level rim of the
tank for gravity flow to be feasible.
iii. Overflow to soakaway pit
• The percolation rate of site soil must be
sufficient to permit infiltration of rainwater
overflows discharged into the soakaway pit.
2. Plan the layout of the overflow system
a) Plan route of overflow drainage piping from the
tank to the overflow discharge location.
b) Contact the municipality and service providers
to ensure that there are no buried service
www.plumbingafrica.co.za