1 October 1997
5-4. OTHER PLANT DISCHARGES.
a. Waste Water. Incinerator plants produce various contaminated waste waters other than
normal sanitary and storm water effluents. These discharges come from water drained from the
waste or used to wash down the tipping floor, water from the ash system, and gas scrubber solution
blow-down. The water from the ash system includes water leaking from the ash collection
dumpsters prior to being removed and emptied. This water may contain toxic minerals leached
from the ash and should be collected and retained in a separate container. Water from the tipping
floor may be slightly acidic and contain significant amounts of bacteria and organic solids. This
water should be processed through a grit trap to collect the larger solids before further processing.
The liquid from the ash system and the scrubber blow-down will typically be alkaline with a high
solids concentration and should be filtered and buffered before release to the sanitary sewer.
Liquids will probably require treatment at the incinerator plant for pH balance and removal of solids
before discharge. Dumping these liquids into the sanitary sewer system without any treatment may
cause problems at the sewage treatment plant. The designer must check with the base sewage
plant and review local water pollution regulations to determine treatment requirements at the
b. Ash. Ash is produced in two forms: bottom ash and fly ash.
(1) Bottom ash is the residue that collects inside the incinerator after the completion of the
combustion process. Fly ash consists of the smaller, lighter particles entrained in the flue gases
that are collected in the pollution control equipment and mixed with used scrubber sorbent. Bottom
ash accounts for 90% (nominal) of the residue and consists mostly of alumina, silica, and oxides of
iron. Bottom ash by itself is alkaline and may be acceptable for landfill disposal without treatment.
(2) Fly ash may contain significant amounts of any low-melting-temperature heavy metals
which were present in the waste stream. The presence of toxic metals will have a significant affect
upon the possible treatment and disposal of the ash. Starved-air incinerators and FBC units
typically retain the greatest amount of heavy metals in the bottom ash because the lower combustor
bed temperatures produce less volatilization. The high-turbulence, excess-air incinerators have the
greatest amount of heavy metals in the fly ash since the higher grate temperatures promote greater
(3) The 1994 decision by the U.S. Supreme Court states that the ash and pollution control
process residues from municipal waste incinerators are not exempt from RCRA (Resource
Conservation and Recovery Act) hazardous waste regulations. This decision now forces all
residual ash to be tested to determine whether or not it meets the criteria for hazardous or
nonhazardous waste. If significant amounts of heavy metals are found and the ash and other
residues fail to meet RCRA requirements, the material will have to be treated as a hazardous waste
and be disposed in a hazardous waste landfill. Disposal in a Subtitle C landfill is considerably more
expensive than disposal in a RCRA Subtitle D landfill.
(4) The current USEPA-approved method of testing for heavy metals (i.e., TCLP) and its
predecessor (EP-toxicity) use different criteria for determining acceptability. The designer should
check which test is currently required. The local pollution control authority will most certainly require
an initial test burn with a typical sample of waste to check for the presence of heavy metals in the
total residue as well as require periodic checks during the operation of the incinerator plant.