Water Journal : Water Journal September 2011
58 SEPTEMBER 2011 water technical features wastewater treatment refereed paper Abstract The rated capacity of a sewage treatment plant has traditionally been simplified to the Average Dry Weather Flow (ADWF). However, the limitation of this tradition is that ADWF is not a process-limiting factor for the design or operation of any significant unit processes. The mass loads of pollutants (eg, COD, BOD, TSS, TKN, and TP) and/or the Peak Wet Weather Flow (PWWF) are the governing factors for the sizing and operational performance of all treatment processes. The benefit of adopting Equivalent Population (EP) to define capacity is that it incorporates these components of hydraulic and mass loads into a single number. It is recommended that wastewater practitioners take up the challenge and adopt EP, rather than ADWF, for defining both the pollutant loads and plant capacity, and that the rated load and capacity should be based on the design and operational limiting parameter in each case. Introduction One of the ongoing challenges for the wastewater treatment industry involves coping with the consequences of water conservation. The benefits of water conservation to the water industry as a whole are understood by both the general public and practitioners within the industry. The basic consequence of lower flows into a sewage treatment plant (STP) may appear superficial; however, it has subtle consequences that can lead to confusion when evaluating the pollutant load entering an STP relative to its nominal design capacity. The rated capacity of an STP has traditionally been simplified to the Average Dry Weather Flow (ADWF), or sometimes as the equivalent population (EP) that is connected to the sewer system. The prolonged drought in south- east Queensland during the past decade, and the consequent water conservation measures that were implemented, have resulted in significant decreases in the flow per capita entering STPs. During this same period there was widespread population growth in the region. The combined effect of these two factors has resulted in many plants having to operate in excess of their design pollutant mass loads, while the influent hydraulic load remains below the design capacity. This situation can lead to confusion when a treatment plant requires an expansion in capacity despite its influent flow being less than the rated capacity in terms of ADWF. Most operators, managers and designers can readily quote the "capacity" of their STPs in terms of ADWF. However, the limitation of this tradition is that ADWF is not a limiting parameter for the design or operation of any significant unit process, other than diurnal flow balancing tanks. The mass loads of pollutants (eg, COD, BOD, TSS, TKN and TP) and/or the Peak Wet Weather Flow are the governing factors for the sizing and operational performance of all treatment processes. While it would be more precise to define STP capacity in terms of "x kg/d COD; y kg/d TSS; z kg/d TKN", this is a mouthful and would be cumbersome and tedious. The benefit of adopting EP to define STP capacity is that it incorporates these components of hydraulic and mass loads into a single number. This concept will be developed and illustrated using a case study of 10 small-to-medium-sized STPs in the Sunshine Coast region. Pollutant mass loads per capita were estimated from an evaluation of influent wastewater monitoring data for Sunshine Coast STPs and were found to be within the expected ranges. The limiting parameter was not the same across all plants, and while flow was limiting at some plants, the organic load (BOD or COD) was more commonly limiting at other plants. Growth rates implied from changes in pollutant loads varied across different STP catchments, as might be expected. However, even within a given catchment, the pollutant loads for all parameters did not increase at consistent parallel rates. Although there are complications and limitations, the simplicity of using a single parameter, namely EP, for defining influent wastewater pollutant loads and STP capacity has merit. MP Thomas Equivalent Population is more sensible than Average Dry Weather Flow DEFINING TREATMENT PLANT CAPACITY Aerial view of the Suncoast Sewage Treatment Plant.
Water Journal November 2011
Water Journal August 2011