Civil Engineering Reference
In-Depth Information
discharge of water treatment wastes to the sanitary sewer system may substantially
increase the hydraulic and / or solids loading on wastewater collection and treatment
facilities. If the wastewater system capacity is adequate, and if the practice does not
negatively impact the wastewater utility's sludge disposal practices, the wastewater
utility may be willing to accept the water treatment wastes. Conditions for such ac-
ceptance often include stipulations on the timing of the discharges, the maximum
discharge rate, monitoring of waste characteristics, etc. In some instances, the dis-
charge of water treatment wastes to the sanitary sewer system may benefit both utilities.
Recycle Concerns
Recycling of the liquid waste streams creates concerns about the
effects on the quality of the finished water. The primary contaminants of concern in
the liquid waste streams are:
Giardia
cysts
•
Cryptosporidium
oocysts
•
Turbidity
•
Disinfection by-products (DBPs), including total trihalomethanes (TTHMs)
•
DBP precursors, including TTHM formation potential (TTHMFP)
•
Total organic carbon (TOC)
•
Assimilable organic carbon (AOC)
•
Taste and odor-causing compounds
•
Manganese and iron
•
Other contaminants may also be a concern, depending on the raw-water constituents
and the treatment processes used.
The introduction of these contaminants back into the main treatment train may upset
process performance, requiring adjustments to the process to achieve a desired finished
water quality. The threat of waste streams to treatment facility performance can also
be hydraulic in nature. If a treatment plant cannot easily adjust the raw-water flow to
compensate for the start of a recycle stream, then the plant flow will increase. This
increase in flow can affect several treatment operations, including chemical feed rates,
process loading rates, the hydraulic profile, and downstream pumping facilities. As
most plants operate at their best under steady conditions, changes in plant operations
can diminish treatment performance. Storage of the recycle flows is often provided so
that these flows can be introduced into the main treatment flow at a controlled rate.
Equalization of waste streams is necessary for at least two reasons: to avoid abrupt
changes in the flow rate and water quality at the inlet to the main treatment plant, and
to minimize the size required and allow consistent operation of any waste stream
treatment facility. Typically, the waste stream flows by gravity to the equalization basin.
Waste stream pumping is then required, either before or after the waste stream treat-
ment process(es), for the flow to reach the head of the plant.
Treatment
To address the concerns associated with recycle flows, they are often
introduced at the head of the treatment process or receive separate treatment before
being introduced into the main flow of the plant.
Typical locations for reintroducing recycle streams to the treatment train are shown
in Figure 23-26. The most common location is at the head of the plant, such as the
inlet to a presedimentation basin or rapid-mix basin. In some cases, recycle streams
