High Purity Oxygen (HPO) activated sludge treatment and nitrogen removal do not have to be mutually exclusive. Some HPO activated sludge treatment facilities in relatively warm climates must address recently implemented or upcoming regulatory changes that require reduction of effluent nitrogen discharge levels. The 48.75 mgd capacity (annual average) Hollywood Southern Regional Wastewater Treatment Plant (Hollywood WWTP) and the 112.5 mgd (annual average) Miami-Dade South District (Miami South District WWTP), HPO activated sludge treatment systems
and are facing these changes. The 96 mgd (annual average) Howard F. Curren Advanced Wastewater Treatment Plant (Curren AWWTP) utilizes high purity oxygen facilities and also has some experience to offer regarding nitrogen removal.
Conversion from HPO to air activated sludge treatment is constrained by significant time, costs and other considerations. From a practical perspective, under certain conditions, efforts can be focused toward other approaches including an “unconventional” approach of partial nitrification and partial denitrification using the existing HPO process, as well as alternate effluent disposal strategies. Generally, experience indicates that low reactor pH, limitations on Solids Retention Time (SRT), relatively high mixed liquor concentrations, increased foaming potential, temperature and other concerns can have significantly inhibit nitrification/denitrification in HPO activated sludge plants. However, recent BioWin modeling and limited field testing at “warm weather” facilities, specifically Hollywood WWTP and Miami South District WWTP, both in Florida, plus a review of some operating data at Curren AWWTP support that, with proper control and appropriate operating conditions, nitrogen removal at HPO WWTP’s can be an appropriate choice.
Effluent disposal flexibility at the Hollywood WWTP offers a relatively simple, pragmatic and cost effective approach to immediate reduction of effluent nitrogen discharges to the ocean. The addition of deep injection wells and the redirection of discharges for primary disposal via the ocean outfall results in an immediate decrease in effluent nitrogen discharge to the ocean of roughly 70 percent. Regulatory policies favor this approach in the short term but will require
even further reduction in the near future. Since effluent nitrogen discharges limits have not been defined, scenarios with increasing levels of nitrogen removal were analyzed using BioWin and with field support data. Scenarios included simple operation changes that increase Solids Retention Time (SRT) and carefully track other key operating parameters, as well as physical modifications and improvements in varying degrees.
Miami South District WWTP detected ammonia in their monitoring wells associated with their deep injection well effluent disposal system. This resulted in a regulatory Consent Order that mandated high level disinfection treatment as well as compliance with the majority of Primary Drinking Water Standards including a nitrite concentration limit of 1 mg/l at this secondary treatment facility. Tests at the Miami South District WWTP confirmed the presence of nitrifiers
in the oxygenation process, that the septage was a contributing source of the nitrifiers and, with an SRT in the order of 3 to 5 days, limited nitrification was achieved. To assess denitrification capability, the Miami South District WWTP oxygenation train fifth stage (fifth out of six stages) was initially converted to an anoxic zone by turning off the aerator in that stage (with occasional bumping for mixing purposes). Based upon this success, aerators in the fifth stage were replaced with relative low energy mixers. The modified oxygenation system was capable of achieving under 1 mg/l of nitrite and about 3 mg/l of nitrate plus nitrite on a relatively consistent basis.