Speaker(s)

H Kenneth Hudnell: Vice President & Director of Science, SolarBee, Inc; Adjunct Associate Research Professor, Institute for the Environment, University of North Carolina

Description

Oxygenation and mixing are required in many municipal and industrial wastewater treatment plants (WWTPs) to improve processing and shorten treatment durations. WWTPs often use mechanical aerators to both oxygenate and mix wastewater. However, much more aeration is needed to thoroughly mix than oxygenate the wastewater. This imbalance creates an operational inefficiency when excessive aeration horsepower (HP) is used for mixing. Excessive HP usage increases electrical grid-power consumption, greenhouse gas emissions and operational costs.

Background

Mixing improves processing efficiency by distributing surface water saturated with dissolved oxygen (DO) and aeration-supplied oxygen throughout the water column, reducing total suspended solids (TSS), preventing thermal stratification and short-circuiting, and homogeneously blending all wastewater constituents. Oxygenation reduces biochemical oxygen demand (BOD) and sludge buildup, increases DO levels, and decomposes hydrogen sulfide and other toxic and malodorous sulfur compounds. Oxygenation also reduces nitrogen levels through nitrification followed by denitrification. Nitrification is the conversions of ammonia to nitrite and then nitrate using oxygen, alkalinity and aerobic bacteria. Denitrification is the conversion of nitrate to nitrogen gas using carbon and heterotrophic bacteria. The nitrogen gas is emitted to the atmosphere, enabling effluent nitrogen-discharge levels to be met.

Goals and Objectives

The New Hampshire Department of Environmental Services established the primary goal of increasing operational efficiency at WWTPs while meeting National Pollution Discharge Elimination System (NPDES) standards. Secondary goals were to reduce grid-power consumption and operational costs while improving or maintaining effluent quality. The Department directed pilot studies at WWTPs in Rochester, Exeter and Pittsfield, New Hampshire, to assess the efficacy of solar-powered circulation (SPC; 1,2) at mixing wastewater and reducing the need for mechanical aeration. Objectives of the studies were to discontinue usage of some aerators after initiating SPC, monitor water quality parameters, sludge thickness and odor events, and calculate reductions in kilowatt-hour (kWh) consumption and expenditure.

Methods and Results

Study site descriptions, HP used for aeration, SPC unit deployment and kWh usage are provided in Table 1. BOD, TSS, DO, ammonia and nitrate levels were systematically measured throughout the study period of April 2006 through June 2007. Malodorous events were monitored throughout the study period, sludge and slurry thickness were measures at the beginning and end of the study period, and cost savings through reduced grid-power consumption were calculated at the end of the study period.

Exeter met all NPEDS limits throughout the study. Effluent levels of BOD, TSS, ammonia and nitrate were similar before and during SPC. DO levels initially dropped below 2 mg/l in all three lagoons when six SPC units and only 87.5 of 327.5 HP aeration were deployed. DO concentration in Lagoons 2 and 3 recovered to > 2 mg/l within 1 and 4 months, respectively, with additional aeration. The additional aeration was discontinued after 5 months. DO was consistently between 0.5 and 1.8 mg/l in Lagoon 1. Odor events were not experienced. Sludge levels decreased in Lagoons 1 and 2, but increased in Lagoon 3, resulting in no net change during the study period. Electrical-grid power consumption decreased by 1,115,295 kWh, resulting in an annualized cost savings of $89,160 and a 3-year payback period.

The results from Pittsfield were similar to those of Exeter; all NPEDS limits were met throughout the study. Aerators were activated as needed to maintain DO levels above 2 mg/l after the 4 SPC units were deployed. Ammonia reductions >95% from June through early November indicated enhanced nitrification. Odor events were not experienced. Sludge levels increased in Lagoons 1-4 by 0.32, 0.12, 0.08 and 0.02 ft., respectively. Annualized electrical-grid power consumption decreased by 260,040 kWh, resulting in a cost savings of $32,037 and a 3.9-year payback period.

Rochester met all NPEDS limits without using mechanical aeration during the study. Pre- versus during-SPC comparisons indicated slight increases in BOD, TSS and sludge, no change in ammonia and nitrate concentrations, and marked improvements in pH and alkalinity. Metals were not resuspended into the water column. Annualized electrical-grid power consumption decreased by 908,292 kWh. Annualized cost savings was $127,449, including $3,760 realized through reclassification to a lower rate schedule. The payback period was 1.5-years.

Discussion and Conclusion

Each of the pilot studies was considered a success; operational efficiency was increased and NPEDS limits were met. Operational costs declined as SPC reduced or eliminated the need for aeration while maintaining or improving effluent quality. Each of the WWTPs continues to deploy SPC. Innovative mixing design powered by solar panels conserves DO, reduces electrical grid-power consumption and operational costs without greenhouse gas emissions.

References

Hudnell HK, et al. Freshwater Harmful Algal Bloom (FHAB) Suppression with Solar Powered Circulation (SPC). Harmful Algae (2009), doi:10.1016/j.hal.2009.10.003 (print form in press).

Hudnell HK. The state of U.S. freshwater harmful algal blooms assessments, policy and legislation, Toxicon (2009), doi:10.1016/j.toxicon.2009.07.021 (print form in press).

Table 1. Study site descriptions, aeration HP, SPC units and kWh usage before & during study

	Total # Lagoons used in study	Total Surface Area (acres)	Total Water Volume (million gallons)	Total Aeration HP in place	# SPC Units Used	Mean kWh/month Pre-SPC1	Mean kWh/month During - SPC1
Exeter2	3	28.1	77.4	327.5	6	166,313	91,960
Pittsfield2	4	5.2	12.3	78	4	50,043	28,373
Rochester3	2	18.3	41.7	140	5	83,400	7,709

¹kWh is all electrical-grid power used at the plants, including that used for aeration
²Partial mix pond system using aeration and mixing in the initial lagoons
³Activated sludge system using aeration and mixing in the raw septage and sludge storage lagoons

Conference

WEFTEC 2010 / 83rd Annual Technical Exhibition & Conference
Ernest N. Morial Convention Center, New Orleans, Louisiana, October 2010