Boehnke and Beyond: A History of Advancements in High-Rate Activated Sludge for Energy Efficient Wastewater Treatment

Introduction
While other two-stage treatment processes had been utilized as early as the 1960s, it was the adsorption-biooxidation (AB) process (Figure 1), developed and patented by Dr. Botho Boehnke in 1978 (German Patent DE2803759C3) that became the most successful, with dozens facilities implemented in Germany, the Netherlands, and other Western European cities. The heart of the A-B process was the initial high-rate activated sludge (HRAS) process, which is a biologically enhanced primary treatment process that can maximize carbon redirection and harvesting from wastewater. The popularity of this technology (and other two stage processes) began to decline in the 1990s as effluent requirements for nitrogen and phosphorus became more stringent (Boehnke et al., 1998). However, efforts over the past decade toward low energy operations and maximization of biogas production, in some cases with the goal of becoming energy neutral, has led utilities and researchers to reinvestigate the potential of HRAS technologies. Such reexamination has led to a much greater understanding of HRAS concepts, including breakthrough technologies and operating strategies going well beyond the original concepts laid out by Boehnke. This paper will provide an overview of the history and evolution of HRAS technologies including the original A-stage concept, as well as further research developments that have led to technology advancements such as the alternating activated adsorption (AAA) process and high-rate contact-stabilization (CS).
A-Stage Process
Boehnke et al. (1997, 1998) first reported A-B process for treating industrial and municipal wastewater in Europe. Based on Boehnke concept, A-stage is defined as a biological primary treatment process (carbon removal) which treats raw wastewater and removes particulates and colloidal matter through extracellular adsorption processes. The A-stage of A-B process by Boehnke et al. includes a short hydraulic retention times (HRT) (30 minutes or less) and SRT (0.125-0.5 d), and high food to microorganism (F/M) ratio (2-10 kgBOD/kgVSS/d) operated at near zero dissolved oxygen (DO) concentrations. Newer Developments on A-stage As noted previously, interest declined in HRAS technologies in the 1990s, but research resumed in the 2000s, which coincided with the greater interest in the industry in energy efficiency and the goal to becoming energy neutral. This included the work by Haider et al. (2003), which investigated the wastewater and biomass characteristics from high-rate systems. More recently, a detailed fundamental study on A-stage (0.3-to-1-day SRT) was examined by Jimenez et al. (2015) to evaluate carbon removal efficiency, carbon capture, impact of DO on removal efficiency and extracellular polymeric substance (EPS) contributions. This development provided leverage on further aerobic A-stage investigations as Jimenez et al. (2015) found that a minimum DO (0.5 mg/L) was necessary for maximum carbon capture efficiency. This finding was corroborated by Cao et al. (2019), which compared aerobic and anaerobic A-stage (similar to Boehnke et al study) performance and found that carbon capture in an aerobic A-stage was 10% higher than A-stage without aeration. Some studies also implemented A-stage with chemical addition for improved bio-flocculation to enhance sludge settleability, which further improved carbon capture.
Alternating A-stage
- AAA One of the most recent advancements of A-stage treatment is the alternating activated adsorption (AAA) process (Figure 1) was first developed by Wett et al. (2021). This technology is a compact design of A-stage system which acts as sequencing batch reactor. The unique feature of this technology is that simultaneous bottom feeding and top effluent discharging provides increased particulate substrate capture efficiency. This technology can be easily applied to existing systems and currently has full-scale installation at the Strass WWTP, Austria and the Rottenburg WWTP, Germany. AlSayed et al. (online in 2021, article issue on 2022) published a detail study on the performance of the AAA process in lab-scale systems treating synthetic wastewater, indicating that this can be a competitive technology for A-stage. However, they reported that the AAA system suffered poor settleability treating low strength wastewater. Bringing Abandoned 'Feast-Famine Technology' to Life – High-Rate Contact-Stabilization While A-stage and AAA technology maximize carbon capture through the biosorption (extracellular adsorption and intracellular storage) mechanism, this phenomenon was first examined by Ullrich and Smith (1951) in a contact stabilization (CS) process in 1951. Later, the development of low-rate CS configuration (SRT > 3 days) was moved forward through several studies between 1975 and 1980 (Gujer and Jenkins, 1975; Benefield and Randall, 1976; Alexander et al., 1980). However, the application of CS process for wastewater treatment gradually decreased in the USA due to its unstable process conditions, and stringent nitrogen and phosphorus limits based on its applied design considerations in that time period (1951 – 1980). From 1980 to 2015, this technology has not received much attention to utilize its promising potential as an HRAS system to recover energy from wastewater. After almost 35 years, Meerburg et al. (2015, 2016), Rahman et al. (2016, 2017, 2019, 2020) and Dolej et al. (2016) studied the fundamental mechanisms of high-rate CS (<2 days) systems in detail by operating bench and pilot-scale studies (Figure 1). Their research also focused on influent total COD mass balance for carbon distribution into different pathways (oxidation, WAS, effluent COD). The establishment of famine/starvation regimes in the 'stabilization zone' was found to promote the adsorption/storage phenomena in the 'contact zone' under feast conditions. A major fraction of the adsorbed organic material is unmetabolized in the contact zone and can thus be captured through WAS as an energy recovery source (Meerburg et al., 2015; Rahman et al., 2016). Ngo et al. (2021) recently reported a successful implementation of full-scale CS for improving clarifier performance and capacity at DC Water's Blue Plains AWTP. The evolution and history of HRAS technologies provide guidelines and recommendations for critical design parameters (Figure 2, 3, Table 1 to 3) that need to be considered for full-scale design.
Conclusions
The full manuscript will highlight key design factors based on the authors' and industry experience for implementing HRAS systems, including: - Operational parameters (i.e., SRT, HRT, DO – aerobic vs. anaerobic) correlating with carbon removal efficiency, redirection and capture from wastewater - Operational flexibility of HRAS systems for downstream nutrient removal technology application - Role of EPS productions in HRAS systems and associated critical factors - Nitrogen and phosphorus removal/redirection from HRAS technologies