A series of variable factors is used in calculating how much
wastewater can be processed by a treatment plant, while meeting provincial
regulations. The numbers limit how many households could be serviced, and
therefore how many new homes could be built.
Here are the main issues for a Town of Erin system, as outlined by
Ainley staff at the June 22 Public Information Centre. They help explain why
the potential urban population for Hillsburgh and Erin village has increased
from 6,000 to 14,559 (over 20-30 years). Erin village could go from 3,225 to
8,565 residents, while Hillsburgh could go from 1,391 to 5,994.
Total Phosphorus
A treatment plant improves effluent quality by lowering
concentrations of suspended solids, E.coli bacteria, ammonia and nitrates, and
maintains desirable levels of acidity and oxygen. But a crucial constraining
factor is Total Phosphorus.
When measured in the river, the existing concentration is .016 mg/L,
well below the provincial limit of .03 mg/L. In what they call a “conservative approach”,
Ainley proposes that the effluent be allowed to increase that concentration to
.024 mg/L. If the target were even higher, it could mean less intensive
treatment, or more homes being allowed on the system.
When measured in the effluent itself, the original strategy in the
Servicing and Settlement Master Plan (SSMP) was to lower phosphorus by a
moderate amount to meet the provincial standard. Total phosphorus in the
effluent would have been .15 mg/L.
The new proposal is to use the best available technology to reduce
phosphorus in the effluent to .046 mg/L, about one third of the previous level.
This would translate to the target of .024 mg/L once the effluent was dispersed
in the river. Ainley President Joe Mullan says this is the most significant factor
in allowing for a higher population to be serviced.
The SSMP predicted a sewage flow of 2,610 cubic metres per day based
on 6,000 urban residents. Ainley is now predicting 7,172 cubic metres per day
for 14,559 residents.
River Flow
The Assimilative Capacity Study (ACS) in the SSMP came up with a
river flow of 202 litres per second during low-flow periods. This measures the
river’s ability to absorb effluent in the driest times, and is a major
constraint.
A revised ACS done by Credit Valley Conservation, with updated river
data, shows a low flow rate of 225 litres per second, 11 per cent higher than
before. This could allow for less intensive treatment, or more new homes.
Water usage and
infiltration
The SSMP used a water use projection of 435 litres per person, per
day. (Water use equates closely to sewage generation.) This is considered very
high.
Based on water use trends in the Town of Erin and a review of design
standards in similar municipalities, Ainley is assuming a sewage flow averaging
290 litres per person per day. They’ve also included an additional allowance of
90 litres per person per day for possible “infiltration” – the water that often
leaks into traditional gravity sewers, increasing the volume that a treatment
plant must handle.
Lower water use per household, and possible reduction or elimination
of infiltration, would both mean that more people could be on the system.
Not connecting some homes
Most residential properties in Hillsburgh and Erin village are too
small for a normal, modern septic system, and so most neighbourhoods would be
hooked up to a sewer system. But three areas with larger lots and newer septic
systems could be exempt from having to hook up.
These are: the Upper Canada Drive subdivision in Hillsburgh, the Credit
River Road – Pine Ridge Road area near the Tenth Line, and the Delarmbro Drive
– Patrick Drive – Erinwood Drive area near Eighth Line and Country Road 124.
Not connecting these areas could free up sewage allocation for new
subdivisions, though this might not be necessary since capacity is increasing
due to other factors. In general, sewers cost less per household if the maximum
number of residents are hooked up and paying a share.
