CASS™ PROCESS DESCRIPTION

CASS™ (Cyclic Activated Sludge System)

U.S. Patents 4663044, 4693821, 4891128, 5013441 and Canadian Patent 524910

BRIEF HISTORY OF SEQUENCING BATCH REACTORS

Activated sludge is the most widely used biological wastewater treatment process in the developed world, treating both sewage and a variety of industrial wastewater. Batch operation of the activated sludge process is nothing new. During the early development of the activated sludge process in the United Kingdom by Ardern and Lockett around 1914, plants were operated using fill-and-draw or batch feed methods. These researchers firmly established the concept of operating a single reactor basin using repetitive cycles of aeration, settlement and discharge of treated effluent. Around 1956, during the development of oxidation ditch technology, Pasveer incorporated interrupted and continuously fed batch treatment principles. Further advancements to the oxidation ditch fed-batch treatment then took place by incorporating a rectangular basin configuration. By the late 1970s, the sequencing batch reactor (SBR) was well established and many small plants were in operation. A major development took place in 1978 with the incorporation of a pre-react selector zone within the SBR to control filamentous sludge bulking. Further refinements of SBR processes took place mainly in Australia and the United States and has led to the wide scale application of the technology worldwide. The shortfalls of the original design have led to the present state-of-the-art CASS™ Sequencing Batch Reactor. While SBRs have generally been classified by the water industry for small or medium scale applications, CASS™ has found application in large scale municipalities (50 MGD / 200,000 m³/d or 400,000+ population equivalent) and the modular expansion, retrofit or upgrading of existing wastewater treatment facilities.

Four BasinCASS™ SBR Plant

Yannawa Multi-Storey CASS™ SBR WWTP, Thailand

The CASS™ SBR is a combination of a biological selector and variable volume process reactor. The process operates with a single sludge in a single reactor basin to accomplish both biological treatment and solids-liquid separation. CASS™ is by design and operation with municipal wastewaters, a biological nutrient removal process, configured to function with filamentous sludge bulking control. A simple repeated sequence of aeration and non-aeration is used to provide aerobic, anoxic and anaerobic process conditions, which in combination with the aeration intensity, favor nitrification, denitrification and biological phosphorus removal.

The essential features of the CASS™ SBR are the plug-flow initial reaction conditions and complete-mix reactor basin. Each CASS™ reactor basin is divided by baffle walls into three sections (Zone 1: Selector, Zone 2: Secondary Aeration, Zone 3: Main Aeration). For typical domestic wastewater treatment applications, these sections are in the approximate proportions of 5%, 10%, and 85%. Sludge biomass is continuously recycled from Zone 3 to the Zone 1 selector to remove the readily degradable soluble substrate and favor the growth of the floc-forming microorganisms. System design is such that the sludge return rate causes an approximate daily cycling of biomass in the main aeration zone through the selector zone. The mechanisms of Zone 1 and the internal sludge recycle eliminate the requirement for separate fill-ratio selectivity, anoxic, and anaerobic mixing periods. The selector is self-regulating for any load condition and operates under anoxic conditions during aerobic periods and anaerobic reaction conditions during non-aerated periods. Polishing denitrification and enzymatic transfer of available substrate during enhanced biological phosphorus removal is also achieved in the selector zone. The complete-mix nature of the main reactor provides flow and load balancing and a tolerance to shock or toxic loading, and the process prevents solids washout during peak or wet weather hydraulic surges.

Typical Two Basin CASS™ SBR Layout

PROCESS CYCLE OPERATION

CASS™ utilizes a simple repeated time-based sequence, which incorporates:

Completion of these three operations in the sequence described above constitute a cycle which is then repeated. The sequence above can also include a FILL, FILL - MIX, FILL - REACT, and REACT if required.

During the period of a cycle, the liquid level inside the reactor basin rises from a set bottom water level in response to a varying influent wastewater flow rate. Aeration ceases at a predetermined period of the cycle to allow the biomass to flocculate and settle under quiescent conditions. After a specific settling period, the treated effluent supernatant is removed (decanted), using a moving weir decanter. This operation returns the liquid level in the reactor basin to the bottom water level. Surplus solids are wasted as required to maintain the biomass MLSS at the required level. Solids wasting after settling enables waste sludge concentrations in excess of 10,000 mg/L to be removed.

FILL - AERATION

The FILL - AERATION (react) operation refers to the air-on time of the process cycle. During this period, influent is received into the basin through the selector zone where it contacts with the sludge biomass recycled from the main aeration zone.

Complete-mix reaction conditions occur in Zone 3 during this variable volume operational period.

FILL - SETTLE

This refers to the first part of the air-off time period when quiescent settling conditions are created in Zone 3 for solids-liquid separation. The activated sludge solids form a sludge-level interface, which progressively falls, towards the floor of the basin. The flocs adhere together and the mass settles as a blanket leaving a clear supernatant.

At the end of the aeration period, the sludge is at a uniform concentration. During the initial settling period, the sludge undergoes internal flocculation due to the residual mixing energy within the basin. As this energy dissipates the sludge interface forms and settles as a blanket. Dense solids fall through the formed mass to settle on the basin floor. There is an initial slow settling velocity, which increases and then gradually falls off due to the compressive accumulation of solids on the basin floor. Zone settling velocity is a function of the initial solids concentration, basin depth, total area of the basin and nature of the biological solids. A top water level solids concentration of 3,500 mg/L will typically settle to form a layer of sludge having a mean concentration of approximately 10,000 mg/L.

CASS™ SBR facilities are sized and configured to operate with inflow into the basin during the settle phase of cycle. Biomass is returned from the main aeration zone to the selector zone to promote selectivity and create anoxic/anaerobic conditions.

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IDLE

In practice, decanting will always be less than the allocated time available. This residual time is designated as IDLE and can be used as a period of inflow without aeration or reaction. The IDLE sequence begins 4 minutes after the decanter has traveled in the reverse up direction and finishes at the end of the designated decant period. Biomass is recycled from Zone 3 to the selector zone to promote selectivity and create anoxic/anaerobic conditions.

RESPIRATION RATE CONTROL (RRC™)

Dissolved oxygen is a necessary requirement for the biological oxidation reactions, which take place with the CASS™ SBR process. Residual dissolved oxygen occurs as a result of oxygen, which is not used by the microorganisms in the biomass. Too much dissolved oxygen in the CASS™ process is wasteful of energy and may inhibit biological nutrient removal mechanisms. A simple control method to ensure optimum biological reaction conditions take place and valuable energy is not wasted. Advantage is taken of the fact that the CASS™ SBR conforms to a complete-mix reaction model. This also means that CASS™ provides a very stable reaction environment when compared to other conventional plug-flow activated sludge, extended aeration, contact stabilization, or sequencing batch reactor systems. A dissolved oxygen sensor is used to measure changes in biomass oxygen demand. For example, a reduction in the oxygen load demand to a CASS™ basin will automatically cause a lowering of the aeration intensity (air supply) so that excessive dissolved oxygen concentrations are prevented.

Conversely, an increase in load demand will cause an increase in aeration intensity so that the metabolic activity of the biomass, as registered by its propensity to use oxygen, is matched with the corresponding aeration intensity rate of air feed into the reaction basin.

RRC™ directly interacts with the best sensor, which is available for the control of air into the process. The system is an in-basin respirometer. Simply stated, low oxygen demand caused by low loadings through diurnal, or other variations can now be directly matched to energy use. The biomass senses the oxygen requirements, which are needed for the process. The dissolved oxygen sensor interprets that message and causes interaction with the rate of introduction of air into the reaction basin.

The CASS™ RRC™ is simple and direct. RRC™ has direct benefits:

SUPERVISORY CONTROL AND DATA ACQUISITION - (S.C.A.D.A.)

S.C.A.D.A. will allow the treatment plant operating personnel to monitor, print and log daily events which occur within the wastewater treatment process functions, as well as the Automated Control Operating functions. This feature allows the operating personnel to randomly extract current and past data information pertinent to the system functionality. The system will graphically display all aspects of the Process Control Equipment and the operating parameters, including set points. Several screen selections including process flow diagrams will enable the operating personnel to view the plant System Monitoring.

The S.C.A.D.A. feature also allows the process equipment supplier, contractor, engineer or owner to remotely monitor, edit, and trouble shoot and repair, if necessary, most aspects of the functionality of the Process Control Center.

350,000 PE CASS™ SBR Facility in China