A seven-month study at a 3MGD facility was conducted comparing the
Ennix process, a novel approach for digestion of wastewater sludge, with a
conventionally operated aerobic digester on a side-by-side basis. The Ennix process does not require aeration
of the digester and employs mixing on an intermittent basis. Periodic additions of selected bacteria and
biochemicals to the waste activated sludge entering a digester are employed to
maintain a favorable digestion environment for sludge biodegradation.
The following conclusions regarding performance of
the Ennix optimization process for wastewater sludge digestion were reached.
· The digester
environment has a significantly lower and less variable oxidation-reduction
potential (ORP) than that of a conventionally operated aerobic digester.
· The dissolved
oxygen concentration is similar to that in a conventionally operated aerobic digester.
· There is evidence
of limited nitrification (and dentrification), accounting for a minor loss of
nitrogen in the process.
· The pH is lower
than expected, given the relatively high alkalinity. This could be due to CO2accumulation, which is evidence of biochemical stabilization of wastewater
transformations result in lower sulfate concentration and equivalent hydrogen
sulfide concentration relative to a conventionally operated aerobic digester.
· The Ennix process,
without aeration and with only occasional mixing, achieves levels of mean
volatile solids destruction and mean fecal coliform removal that are equivalent
to a conventionally operated aerobic digester.
· Sludge metal
contents are not changed appreciably in the process relative to input waste
activated sludge or a conventionally operated aerobic digester.
· The process can
meet EPA 503 Class B biosolids requirements for indicator organisms and sludge
metal contents at a relatively low degree-day value of 483 °C d. Testing for vector attraction
reduction was inconclusive due to digestion of extended aeration sludge and
provision of a low value of degree-days.
· $26,500 could be
saved annually at the 3 MGD test site by using the Ennix digester optimization
procedure in lieu of conventional aerobic digestion, while achieving equivalent
· Savings of $200 to
$400 annually per 1000 ft3 of digester volume are expected using the
Ennix optimization procedure instead of conventional aerobic digestion. These values are based on 2% solids digestion
and 6.5 cents per kWh.