ABSTRACT Sequencing batch reactors (SBR) are a flexible configuration of the activated sludge process, capable of providing low effluent nutrient levels. The SBR is fundamentally the same as any other variation of the activated sludge process, except that it operates in a batch draw and fill mode. The nuances of a batch operation must be understood and applied. This paper presents design considerations unique to the SBR and a design comparison of a continuous flow system and SBR, demonstrating that the total volume in a SBR system is not less than a comparably sized continuous flow system and that the total number of diffusers and installed blower horsepower is greater for the SBR system. KEYWORDS
Sequencing batch reactor, SBR, activated sludge, aeration
INTRODUCTION Sequencing batch reactors (SBR) have become a popular means of providing activated sludge treatment. With proper design and configuration, SBRs will provide very low effluent nutrient levels. The SBR is fundamentally the same as any other variation of activated sludge—the same concepts, design procedures, and biological kinetics are applicable. However, the batch operation of the process and impacts on design aspects must be understood and correctly applied to achieve optimum results. In particular, the batch operation has a significant impact on sizing of the aeration system. This paper presents a sizing comparison between conventional flow activated sludge and a SBR system with varying numbers of reactors, demonstrating the impact of the number of reactors on the sizing of the aeration system. The paper also discusses the impacts of alkalinity control, decanting, the necessity of scum removal and advantages of the SBR compared to continuous flow systems. SEQUENCING BATCH REACTOR – DESCRIPTION AND DESIGN SBRs are not a recent innovation. Activated sludge was developed as a batch process in the early 1900s. The first full-scale activated sludge plants were variable volume, batch operated systems (Wilderer, et al.) (EPA, 1999). All of these fill-draw plants were converted to continuous flow operations by the mid-1920s, even though fill-draw operation was known to produce a higher quality effluent (Wilderer, et al.) Reasons for the conversion were: 1. Due to lack of instrumentation and automatic controls, all operational changes were manual. 2. The coarse bubble diffusers used for mixing and aeration were subject to clogging during the settling phase, requiring manual cleaning. 3. Discharge rates from the fill-draw operations were greater than influent rates. Convenience and cost were given preference over treatment performance and continuous flow operation became the standard practice (Wilderer, et al.). In essence, the batch systems were not operable when applied to a full-scale application of any significant size. It is interesting to note that sludge bulking common to almost all activated sludge applications prior to the recent use of selector technologies was not noticed until the advent of continuous flow systems.
Fill-draw systems re-emerged in the Netherlands in the 1960s, with limited success, and in Australia in the 1970s. With the development of computer logic controlled instrumentation and increased aeration efficiency, fill and draw systems became a viable activated sludge alternative. Batch or fill and draw systems provided in multiple units are now termed sequencing batch reactors. Current SBR technology produces effluent quality equal or superior to conventional continuous flow systems. Any wastewater that can be treated by continuous flow activated sludge can be treated to similar levels by properly designed and operated SBRs. Aeration and clarification are done in single tank operating on time controlled cycles. During the first phase of the cycle the unit...