Blower Turndown: How Low Can You Go? And Is It Low Enough?

BACKGROUND
Our industry is moving towards more environment-friendly designs, taking up the challenge of net zero energy and carbon footprint reduction. It has been shown (SAIC, 2006) that aeration in the typical activated sludge plant consumes approximately 50% of the total power demand. Recently, we have seen the implementation of new blower technologies as well as new aeration control strategies focused on saving power. Some strategies, such as ammonia-based aeration control, strive to may reduce oxygen demand by allowing simultaneous nitrification-denitrification while also improving effluent quality. However, if the blower system has insufficient turndown to accommodate the full range of required airflows, these new strategies will not achieve the full potential savings. This paper investigates the common causes of overestimating minimum airflows and identifies the factors that need to be considered to more accurately determine the blower turndown requirements.
METHOD
When designing aeration systems, designers tend to make conservative assumptions, that maximize the estimated blower capacity. Most parameters are assumed to have constant values for a given design. However, some variables are subject to diurnal and/or seasonal variation: standard oxygen transfer efficiency (SOTE) for the diffusers, the alpha factor (alpha), mixed liquor and air temperatures. A dynamic process model for a biological nutrient removal plant treating 14 ML/d (3.7 mgd) was used to estimate the diurnal oxygen demand for a typical day. The required blower airflows were determined for the hottest day of the year, using hourly climatic data. The calculation was repeated for the coldest day of the year. Finally, the airflows for the cold day at startup was calculated, while adjusting alpha and SOTE. The impact of other design conditions, including maximum month, peak day, minimum day, etc. will be discussed in the manuscript. The SOTE for any fine bubble diffuser increases with reduced airflow and under reduced loads, as reflected in the curves from diffuser suppliers. SOTE depends on several factors, including diffuser submergence depth, layout and the allowable operating range for the selected diffusers. The designer needs to carefully adjust the number of diffusers to balance all these factors. Of all the aeration parameters, alpha is the least understood. It is surprising that it is so seldom measured, considering its importance to aeration system design and operation. Off-gas testing has been used for decades to determine the value of alpha, but only recently has it been measured over extended periods, so that diurnal variations in its value could be established. Trillo et al (2014) showed that in an oxidation ditch equipped with fine bubble diffusers, alpha varied by a factor of about 1.5 over a single 24-hour period (see Figure 1). Sosa Hernandez et al (2019) showed that during constant feed flow in a rectangular BNR aeration basin, alpha may vary by a factor of 2.0. Under typical diurnal load variations, excluding flow equalization, the change increased to a factor of 2.5. Using previously published data, Jiang et al (2017) developed a negative correlation between local COD concentration and alpha. Rosso and Stenstrom (2005) showed that alpha is a function of operating solids retention time, that it increases along the flow path in the aeration basin and that adding an unaerated selector upstream of the aerated zones increases alpha. This highlights the inadequacy of assigning a single value to alpha during design. Clearly, the need to assess the effect of variations in alpha will be critical in identifying the full range of airflows that a blower system will need to provide. The wastewater temperature will vary seasonally. At most plants it would be reasonable to assume that peak design OTR could coincide with maximum wastewater temperature and minimum design OTR with minimum wastewater temperatures. This would again tend to move the minimum and peak design airflows further apart. Many design engineers would take the analysis no further than the normalized or standardized airflows (Nm3/h or scfm). Blowers, however, are constant volume machines and the normalized or standardized capacity of a given blower changes with inlet air temperature and humidity. Using only normalized airflows implicitly assumes the blowers will always receive air at one temperature and humidity, typically the maximum design temperature and humidity used for determining the maximum blower capacity. For the purpose of illustration actual blower airflows are used in this paper. The minimum airflow to a specific zone may be set by mixing requirements. Using an overly conservative mixing airflow may result in the airflow to a given zone set by the mixing requirement for the bulk of a typical day.
RESULTS AND DISCUSSION
The impact of all these factors can be illustrated by an example. Figure 2 shows the actual blower airflow requirements. The figure shows that if a constant alpha value is used (orange line), the result would tend to underestimate peak flows while overestimating the minimum flow compared to variable alpha (gray line). Both lines are based on the hottest day in summer. When the coldest day in winter is considered (yellow line), the entire airflow is lower, so the minimum airflow is 14% lower for a location such as the northeastern part of North America where both hot summers and cold winters are the norm. The figure also illustrates the impact of startup conditions: while the startup flows and loads are 23% below those of design, the minimum airflow is 33% lower, due to higher alpha and SOTE values at startup. The minimum diurnal airflow required at startup is less than 20% of the maximum airflow, while the minimum oxygen demand is 33% below maximum. Using constant SOTE, alpha and air temperature would overestimate the minimum airflow by 73%! Airflow to aerated zones are sometimes maintained at a certain minimum flow to ensure mixing. The consequence of setting such a minimum is that the DO concentration would increase above the process setpoint when oxygen demands are low. For some systems, particularly those aiming to achieve enhanced nutrient removal, high DO might be more detrimental to the process than a small amount of settling. Pretorius et al. (2015) reported that an air flux 50% below the commonly accepted minimum mixing guideline (0.12 scfm/sf or 1.1 Nm3/m2.h) was sufficient. Anecdotal observations from other WRRFs support this observation, suggesting the guideline can be lowered and tailored for each application. Different blower technologies offer different turndown ratios, and the designer must consider these and the implications for the number of units required. The manuscript will include a full discussion of blower selection.
CONCLUSION
Our analysis shows that assuming constant values for certain variables can lead to a significant underestimation of the required downturn, leading designers to overestimate the actual minimum downturn by over 70%.