Sustainability Through Biotechnology TM

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 sludge.


·  Sulfur 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 performance.


·  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.