Converting Rectangular and Circular Primary Tanks Into An AAA Settler

INTRODUCTION
As water resource reclamation facilities (WRRFs) look to upgrade or expand carbon removal or nitrification only facilities, they have two paths, one that needs larger biological tank volumes and more energy and to expanding upon existing treatment concepts, or, to otherwise develop a virtuous cycle of technologies that both reduce footprint and intensify treatment but also need less energy and use the chemical oxygen demand wisely. The key to achieving this virtuous cycle is optimizing the first major treatment step of wastewater treatment. Most conventional plants in the United States have a 'primary process' with clarifiers for TSS removal followed by a secondary process for carbonaceous BOD or COD removal to achieve TSS and cBOD5 limits of 30 mg/L. Our value proposition is to enhance the primary treatment step to an alternating activated adsorption (AAA) settler instead to
1) make better use of this infrastructure within the existing hydraulic profile,
2) intensify the biological step,
3) use less energy in the biological step,
4) to produce more gas in a digester,
5) produce 5% thickened solids and
6) replace cumbersome primary mechanical equipment such as chain and flights with air lift sludge withdrawal Carbon Redirection: The goal of the AAA settler is to convert influent soluble COD and ammonia through assimilatory means to particulate heterotrophs in a very compact design (0.5 days solids residence time and two-hour hydraulic retention time) to achieve both carbon and nitrogen redirection. The famous 'Strass' case study of energy positive BNR facility exemplifies this virtuous cycle brought about by the A-stage process combined with deammonification (Wett et al, 2004.). The problem for most A-stage processes is that they are 'bioflocculation' limited (Jimenez et al, 2015) and unable to properly trap this nitrogen containing heterotrophic organisms in a colloidal state. Any process that could improve colloid capture at the same SRT and surpass A-stage performance could provide savings through intensification, aeration and more methane gas production and more resilient series/parallel operations. The rethinking of this primary clarifier removal has been variously proposed with many approaches such as chemically enhanced primary treatment, primary microfiltration sieves, primary fabric filters and contact dissolved air flotation treatment being considered to decrease the carbonaceous inventory to the downstream secondary or biological nutrient removal (BNR) step. The problem with such treatment steps is that they are 'add-on' physical processes that focus on carbon redirection only. Instead, the conversion of the primary into a AAA biological settler can also provide wet weather parallel secondary treatment thus possibly addressing 'blending' considerations. Thus, an optimized 'biological primary' concept should remove and redirect not only carbon but also provide an opportunity for 'parallel' wet weather operations, and 'series' dry weather operation modes to allow for resilience and flexibility in removal of soluble COD. Objectives: The goal of this paper is to describe the design, commissioning and operations of the AAA settler as implemented in 4 plants in Europe and in design in 10 additional plants. The paper will describe the implementations in both rectangular and circular configurations. Mass and energy balances will be provided to help engineers understand opportunities to realize both energy savings and space savings to enhance treatment capacity.
METHODOLOGY
The AAA settler uses a floc-filter to improve colloid capture by feeding the influent at the bottom while withdrawing the treated effluent at the top in a primary converted into a constant water level reactor with an alternating feed/withdraw cycle that spans half the time and a react/settle/waste cycle that spans the other half of the overall cycle (see Figure 1). The AAA settler needs a minimum of two primary clarifiers in parallel that are converted to sequenced operation. The primary clarifiers can be either circular or rectangular to achieve this conversion. Fine bubble diffusers are installed at the bottom of the tank to provide both air and mixing. No mechanical equipment is needed and the existing and often troublesome sludge scraper equipment is removed. A replacement sludge withdrawal manifold using an air lift system is used to remove the settled sludge and wasted to an adjoining gravity thickener. Thus the effluent half pipe, process reactions, and the sludge withdrawal are managed using the same blower/air system considerably simplifying operations. A gravity thickener is installed in the inner conical section of a circular primary or in the sludge hopper of a rectangular primary to achieve thickening to 5% or greater solids content. Figure 1 shows the schematic of this process with alternating operating cycles and the bottom feed piping.
RESULTS Figure 2 shows the performance of the AAA process in Rottenburg, Germany with influent and effluent COD and Nitrogen (left y-axis) and 67% COD removal and 30% nitrogen removal. This high removal in the absence of any chemicals is attributed to the floc-filtration phenomenon where the colloids that contain mostly heterotrophic organisms that have nitrogen containing proteins and usually escape in the effluent are now trapped within the sludge blanket of the AAA process. This enhanced removal is a key feature of the AAA process. Figure 3 shows a Sankey diagram comparing a conventional primary + BNR plant, an A-stage + BNR plant, and a AAA + AvN controlled BNR plant. The figure shows A-stage 54% COD removal and 22% nitrogen removal with a downstream COD/N ratio = 6.7. The AAA removes 61% COD and 30% N with a downstream COD/N ratio = 6.1. The secondary volume requirements with a AAA settler is roughly 50-60% of the volume required with a conventional primary, resulting in a very compact overall process that can be seamlessly employed for nitrification, phosphorus removal or nitrogen removal permits.
CONCLUSIONS
There are now 14 AAA settlers in various stages of implementation with 4 fully operational in both circular (one plant at Strass, Austria, Figure 4) and rectangular configurations (three plants operational) in Europe. The AAA settler is now one of the fastest growing new technologies in Europe, doubling every 0.5 years, as simultaneous concerns of urban growth and climate change mitigation increase its attractiveness to utilities. The AAA settlers uses existing primary tank hydraulic profile and infrastructure (of two-hour hydraulic retention time) and converts it into a constant water level process, sequenced contact stabilization biological process with no mechanical equipment and with an included gravity thickener. The full presentation will describe the learnings from these conversions with particular attention paid to regulatory drivers from a systems perspective for design.