Did you know that wastewater contains ten times the energy needed to treat it? Located near Strass im Zillertal, the Strass wastewater treatment plant (WWTP) serves 31 communities in the Achental and Zillertal valleys east of Innsbruck, Austria. It provides wastewater treatment for a population that ranges from approximately 60,000 in the summer to 250,000 during the winter tourist season, and has treatment requirements that include organic and nitrogen removal. An energy-independent facility, the plant produces more electrical energy than it requires for its operation.
The peak winter flow and load is equivalent to a plant treat- ment capacity of 10 mgd. Using a two-sludge system (high rate BOD removal followed by nitrification/denitrification), it provides for both nitrogen and phosphorus removal, biologically and chemically, respectively. The plant was commissioned in 1999 and successive optimization efforts over the past decade have resulted in significant cost and resource reduction. Highlights of these efforts include:
- Reduction of chemical costs for sludge thickening by 50%.
- Reduction in energy consumption on mass treated basis from approximately 6.5 euro/kg NH4-n removed in 2003 to 2.9 euro/kg NH4-n removed in 2007/2008, primarily through active management of dissolved oxygen (DO) setpoints and conversion of the aeration system from conventional fine bubble to ultra- high efficiency strip aeration.
- Reduction in energy consumption for sidestream treatment from 350 kwh/d to 196 kwh/d by implementing a novel sidestream nitrogen removal system (DEMON®).
- Enhanced utilization of the digester gas by converting to a state-of-the-art cogeneration unit, boosting electrical efficiency from 33% to 40% and overall usage efficiency from 2.05 to 2.30 kwh/m3 of digester gas.
Treatment Processes Operated at the Strass Plant
The Strass plant provides two-stage biological treatment (A/B plants) to treat loads varying from 60,000-250,000 population equivalents (weekly average), with higher loads during tourist seasons. The high loaded A-stage with intermediate clarification and a separate sludge cycle eliminates 55-65% of the organic load. The A-stage is operated at 0.5 day sludge retention time (SRT), while the target SRT in the B-stage is about 10 days. Nitrogen elimination in the low-loaded B-stage is achieved by pre-denitrification to produce an annual N-removal efficiency of about 80% at a maximum ammonia effluent concentration of 5 mg/L. All activated sludge tanks can be operated aerobically if required. Air-flow and aeration periods are controlled by on-line ammonia measurement.
All excess sludges are thickened, anaerobically digested, and dewatered. The sidestream from dewatering is treated prior to re-introduction into the main plant processes using Sequencing Batch Reactor (SBR) technology adapted for ammonia removal using the DEMON® process. Figure 3 summarizes the chemical oxygen demand (COD) balance among the main subsystems of the Strass plant.
The two-stage biological treatment approach results in the high-rate entrapment of organics without excessive aerobic stabilization in the A-stage system. Due to the reduced SRT, organic compounds are removed mainly by adsorption from the A-stage onto solids and are immediately conveyed through thickening and digestion, where the conversion of organics to biogas occurs.
The Transformation from Energy Consumer to Producer
The annual rate of energy consump- tion in 2005 was 7,860 kWh/day. The electricity demand of the B-stage represents 47% of the total consumption. Due to site constraints, the Strass plant has relatively high energy consumption rates for influent pumping (9%) and for off-gas treatment (13%).
Air supply to the B-stage biological process is primarily governed by nitrification requirements; air supply is not required for heterotrophic nitrate reduction in anoxic zones of the biore- actor. The Strass WWTP employs swing zones that can be used in either aerobic or anoxic modes, alternating to minimize air supply and energy requirements. The reactor volume required for aerobic nitrification is adjusted to maximize the denitrification volume while still achieving full nitrification, which depends on the instantaneous actual load. Intermittent aeration of the swing zones is operated between two set-points of the on-line ammonia control, leading to extended aerobic intervals in the afternoon. If the ammonia concentration increases to a maximum threshold value, then all of the swing zones are aerated rather than used for denitrification. This control strat- egy results in stable ammonia removals and fluctuating effluent nitrate concentrations.
Electricity production by the biogas-driven generators was 8,490 kWh/day in 2005. The collective result of many individual measures, the percentage of energy self-sufficiency improved steadily from 49% in 1996 to 108% in 2005. A major step forward in energy production was the 2001 installation of a new, higher-efficiency, eight-cylinder, co-generation engine that provides 340 kW of power. The new co-generation units have an average conversion efficiency of 38%, which is 20% higher than the 33% efficiency provided by the previous units.
From 1997 until 2004, Strass operators applied an SBR strategy for nitrification/denitrification using excess sludge from the A-stage system as a carbon source. After 2004, the plant implemented the DEMON® process for deammonification, which does not require supplemental carbon (Wett, 2006). This achieved two favorable outcomes: energy requirements for nitrification of the sidestream ammonia were reduced, and the organic sludge previously required for denitrification of the sidestream was now available for conversion to biogas within the digesters. The higher proportion of A-stage sludge in the feed to the digesters increased the methane content from about 59 to 62%. The combined benefits of the savings in aeration energy and additional methane result in an overall reduction of 12% on the plant-wide energy balance (Wett and Dengg, 2006).
Source: Water Environment Research Foundation: Case Study: “Sustainable Treatment: Best Practices from the Strass in Zillertal Wastewater Treatment Plant.”, www.werf.org