Residuals Management - Public Water Systems - page 797

The following conclusions about discharge of WTP residuals to waterways came
from the 1996 USEPA Handbook on Management of Water Treatment Residuals: 1
Because of the potential toxicity to aquatic organisms, utilities that discharge WTP resid-
uals to soft waters (with hardness less than 50 mg CaCO 3 / L) should consider a different
residuals disposal method. In addition, receiving waters with a pH less than 6 should be
avoided. Discharge to these waters could result in the increased solubility of metals and
increased toxic effects.
Discharge to Sanitary Sewers In 1968 8.3 percent of WTPs discharged solids to
sanitary sewers. 71 A 1979 survey of alum users found that 27 percent of these plants
discharged sludge to the sanitary sewer, 66 so it is increasing, but a 1981 survey of
softening plants found that only 8 percent of these plants discharged softening sludges
to sanitary sewers. 12 Softening plants produce greater solids quantities, which are a
larger load on the wastewater treatment plant process—a fact that may account for
their more limited use of this method.
This technique of sludge disposal transfers the solids-handling problem from the
water treatment plant to the wastewater treatment plant (WWTP). However, inclusion
of the necessary capabilities in the solids-handling facilities of the WWTP may result
in an overall cost savings, by consolidating the equipment and reducing the number
of personnel required for total solids handling. Many wastewater utilities are concerned
that the water treatment plant solids will adversely affect their treatment processes.
However, these same chemicals are used extensively in waste treatment to remove
phosphorus, and no adverse effects result. A number of factors must be evaluated if
this approach receives serious consideration for a given application.
A major consideration is the ability of the wastewater collection system and WWTP
to accept the increased hydraulic and solids load imposed by the addition of the WTP
wastes. The direct discharge of spent-filter backwash water into the sewer system, for
example, could cause a hydraulic overload of the collection system, or a hydraulic
surge large enough to cause the WWTP clarifier performance to deteriorate. Hydraulic
surge storage at the WTP, with gradual release, may be needed if the volume of water
plant waste is large in proportion to the wastewater flows. Release during low waste-
water flow periods (midnight to 6:00 A . M .) may be desirable. Another aspect to con-
sider is that the sewer receiving the WTP sludges must be of adequate capacity and
should provide velocities adequate to prevent deposition of the WTP sludge in the
sewer. Studies at Detroit report that a velocity of 2.5 ft / sec (0.76 m / s) is adequate to
prevent settling of the sludges in the sewer. 46
The bulk of the solids from the WTP sludges will be removed in the WWTP
primary clarifier. The solids-handling system at the WWTP must be capable of han-
dling the additional solids load. It will be a rare and fortunate circumstance for an
existing WWTP solids handling system to be able to handle the unplanned addition
of WTP sludges if the water plant and wastewater plant are of comparable size. Dallas,
Texas, in an attempt to dispose of waste lime sludge and to use it effectively for
wastewater treatment, found that it greatly improved the removal efficiencies of both
BOD and suspended solids. 72
Culp and Wilson in 1979 studied the effect of adding alum sludge to an activated
sludge wastewater treatment facility and reported no significant benefit or detriment
to the treatment process or the anaerobic digester. 73 There was an increase in sludge-
handling quantities in proportion to the increased water treatment solids.