By Dr. Robert A. Governal

Summary: By minimizing mechanical components and simplifying system controls, a low maintenance plant can be cost effectively designed and easily operated and maintained to manage waste water recycle opportunities, including biological nutrient removal (BNR). A compact footprint as well as elimination of yard piping is generated through the use of common wall tank design. Mechanical pumps are virtually eliminated from the plant through use of compressed air and gravity as key driving forces in fluid transport.

With increasing global population, dwindling natural resources and tighter laws regarding effluent discharge quality, the recovery and reuse of the world’s wastewater is rapidly becoming a paramount issue for industries, municipalities and entire countries.

Recent case studies2,3 have verified the increased importance and demand to recover and reuse treated wastewater. Water is currently viewed as “the single most critical natural resource in the Hashemite Kingdom of Jordan,” as this desert country receives little annual rainfall and the its rugged geography can’t adequately replenish groundwater supplies.2 Zero-discharge requirements—the policy of protecting a water supply by minimizing the amount of unused effluent streams a treatment facility generates—mandate present and future wastewater plants’ effluent and solids be either reused, disposed of in earthen evaporation basins or recycled back into the plant.

In Ciudad Puebla, Mexico, five new wastewater treatment plants have been proposed in a plan to reduce the risk of cholera.3 The effects of decades of raw industrial and municipal wastewater discharges into the rivers that are sources of crop irrigation and hydroelectric power have resulted in water quality described as “foaming, stench, grayish color, and lack of aquatic life.”

These two examples represent only a small fraction of global opportunities for novel wastewater treatment technologies for small systems that can be addressed by a combined sequential oxidation and clarification process.

The process
The sequential oxidation process, when combined with clarifier technology, is a batch nutrient removal process that continuously produces clarified effluent. Greatly reduced concentrations of contaminants from raw wastewater environments result from the control and optimization of key environmental parameters including dissolved oxygen concentration, oxygen transfer rate, limiting bacterial nutrient levels and biomass settling velocity. These processes are applicable and cost effective to both municipal and industrial wastewater environments, with system flow rates up to several millions of gallons per day (mgd).

An example of such a system design is shown in Figure 1. The raw wastewater enters the selector tank, where it combines with return activated sludge (RAS) from the clarifiers. The RAS stream contains a high concentration of settled biomass that has already been optimized to degrade contaminants in the system. The function of an RAS stream, when recycled to the selector tank, is to assist the tank in providing an environment to promote growth of optimal bacterial organisms (heterotrophic Gram negative rods such as Zooglea4 species), while discouraging growth of undesirable filamentous bacteria by introducing a high concentration of beneficial organisms into the tank. Filamentous bacteria, which may include Gram positive species4 such as Mycobacterium, Streptomyces or Sphaerotilus, can form a high surface area to volume ratio floc that displays poor settling characteristics, and poor contaminant conversion.1 The selector tank also provides a stable environment to dilute any shocks (toxic contaminants) that can surge into the system and damage existing microflora. The continuously aerated selector provides a well-mixed tank with a residence time typically of 15-to-30 minutes.

The water then flows into the continuously aerated first stage basin where the majority of biological oxygen demand (BOD) and ammonia removal occurs. The system design ensures every drop of water will undergo complete aeration.

The flow continues into the second stage where nitrification (ammonia oxidized into nitrate) is achieved by sequencing air in a series of “on” and “off” cycles. When the air is “on,” BOD and ammonia continue to be removed from the well-mixed system through aerobic microbial decomposition of contaminants. When the air is “off,” sludge quickly settles to the bottom of the tank, the dissolved oxygen in the water is depleted (the environment quickly becomes anoxic, or deprived of oxygen), the bacteria’s metabolism shifts and denitrification occurs—converting nitrates to nitrogen. When the air is turned back “on,” the bacteria absorb more phosphorous than is needed for normal metabolism, a phenomenon known as “luxury phosphorous uptake.” The water undergoes several oxic/anoxic cycles in the second stage aeration basin. A noteworthy feature of a sequential oxidation system is the ability for it to sequence air flows from one tank to the other without having to stop the blowers since sequencing is controlled by valves.

The water then flows through a series of screens into a unique clarifier technology with one of the lowest entrance velocities of any industrial technology. The low entrance velocity generates a minimum of fluid turbulence and promotes rapid settling of solids. The clarified effluent is continuously drawn from the top of the clarifier tanks, while the activated sludge is returned to the selector tank via airlifts and a return trough.

Flow regulation
Peak flows are handled within the plant via an enhanced flow regulation system to prevent contamination of clarified effluent by solids. The built-in surge control equipment operates via a triple weir device located inside the effluent boxes in the clarifier. The triple weir device controls the rate at which the clarifier compartment will pass effluent by capping the water’s upward velocity in the clarifier. When a peak flow occurs, the excess quantity of incoming wastewater is conveniently stored in the aeration tank within a predetermined freeboard. This peak storage delays the processing of a portion of the mixed liquor suspended solids (MLSS), or partially treated process fluid, by the clarifier and allows the effluent flow rate to be controlled below the maximum design surface overflow rate (SOR).

The MLSS is important to the process, as it’s a direct measure of the amount of biomass in the system available for contaminant removal. The first weir sets a minimum level for the clarifier and freely passes effluent flow. The second weir is a submerged adjustable swing gate orifice and freely passes any flow until the rated capacity is approached. At this point, the first weir becomes submerged and the second weir takes over flow control, restricting effluent flow to insure the maximum SOR isn’t exceeded. If a prolonged or abnormally high inflow occurs, a maximum level in the plant may be reached. At this point, the third weir regulates the effluent overflow and bypasses the first two weirs avoiding further storage. When the influent flow rate subsides enough to drop the plant level below its maximum, the remaining stored “surge MLSS” is processed at a normal SOR. The surge control system can effectively dampen high daily flow peaks and lift station surges without exceeding the maximum clarifier SOR, and it insures a high quality effluent.

Although flow equalization is inherent in the design, surges may be of such long duration as to exceed storage capacity within aeration tanks. At that time, as with any clarifier, a hydraulic overload could be expected, requiring some form of separate flow equalization. In this case, the MLSS is directed by a bypass weir to an auxiliary surge basin before water levels exceed the high level weir. At the same time, float-operated surge return pumps within the sideline basin sense the presence of the mixed liquor, are activated, and recirculate mixed liquor back into the aeration basin during the peak flow period. Since good recirculation is maintained within the tank, solids in the sideline basin remain in suspension and the mixed liquor remains freshly aerated. Eventually, the raw sewage inflow will subside and—as the clarifiers continue to process the mixed liquor at their design rate—then the recirculation pumps overcomes the overflow rate and the sideline surge tank empties down to the sump.

Within a sequential oxidation plant, all flows are driven by differential head (gravity), so no mechanical pumping is required within the water treatment plant. Waste activated sludge (WAS), process fluid lifted from the aeration basin and sent to the digester, and RAS (process fluid lifted from the floor of the clarifier and directed into the selector tank) flow between tanks are transported by airlift pumps. The entire plant, therefore, is run using compressed air.

Design criteria
Essential design criteria can include:

  • Influent source (municipal or industrial),
  • Hydraulic conditions (hydraulic loading, peak hourly flow in mgh, peak daily flow in mgd and inflow period in hrs/day),
  • Influent quality parameters (average, maximum and minimum for both summer and winter, if available) that include BOD, total suspended solids (TSS), chemical oxygen demand (COD), NH3, pH, phosphate (PO4-2) concentrations in milligrams per liter (mg/l),
  • Effluent quality desired (which can also include a dissolved oxygen and fecal coliform specification),
  • Wastewater temperature ranges,
  • Site elevation (necessary for blower calculations),
  • Power available (volts, phase, cycles in hertz or Hz),
  • Sludge age (days),
  • Retention time (hrs), and
  • Digester volume in gallons (gal).

In many cases, as with any application, some or most of the parameters listed may not be known at the time of initial design. The engineer, therefore, must be experienced enough to know and present reasonable assumptions within the design criteria. Critical design points in the sequential oxidation system to achieve enhanced nitrification therefore include the aeration tanks, blowers and air diffuser types.

Maintenance is typically confined to the plant proper, and consists of normal blower maintenance, periodic system washing and diffuser cleaning. It should also be noted the entire plant is controlled through use of timers—no computers are required. As such, every wastewater treatment plant designed in this manner is thus Y2K compliant.

Although influent contaminant conditions (BOD/TSS/NH3/P levels in mg/L) can vary from 250/250/40/5 to 350/350/80/10 and beyond for many municipal applications, most effluent conditions are designed to fall in the range of 10/15/1/1 mg/L or below. Chemical feeds can flocculate sparingly soluble ions and decrease phosphorous levels to parts per billion (ppb) or micrograms per liter (µg/l) concentrations.

Since high system residence times (SRTs) can make filamentous bacteria growth a potential problem, the activated sludge that settles to the bottom of the first stage tank is periodically wasted to the aerobic digesters to maintain a young and active microbial population within the tank. Solids in the aerobic digester (1-to-2 percent solids, typically) are periodically withdrawn to trucks for disposal or they’re dewatered (filter press, filter bags, etc.) to a level of up to 20 percent and can potentially be disposed of in a landfill as a reduced volume non-hazardous solid waste.

This paper has been an introduction into the sequential oxidation process. Future work in this area will show how modifications in design parameters can affect capital and operational costs, design layout, and data resulting from real world operations.


  1. Bailey, J., and D. Ollis, “Chapter 12: Biological Reactors, Substrates, and Products II: Mixed Microbial Populations in Applications and Natural Systems”, in: Biochemical Engineering Fundamentals, McGraw Hill Book, Co., New York, pp. 712-713, 1977.
  2. Mansour, M., and F. McNeill, “Middle East Thirst, Wastewater Reuse Is Key to Solving Jordan’s Water Problems,” Water Environment Technology, 10(11), pp. 44-48, November 1998.
  3. Ortiz-Juan, R., M. Sailor, and G. Barnetche, “Performance Specifications, A Mexican Case Study in Design-Bid Evaluation,” Water Environment Technology, 8(7), pp. 52-56, July 1996.
  4. Atlas, R., “Environmental Quality: Biodegradation of Wastes and Pollutants,” in Microbiology: Fundamentals and Applications, Macmillan Publishing Co., 1984, p. 863.

About the author
Robert A. Governal, Ph.D., is a regional sales manager with Waterlink/Aero-Mod in Manhattan, Kan. Governal earned his doctoral degree in chemical engineering from the University of Arizona. He has written over 35 technical publications and holds one patent in the field of water purification technologies. He can be reached by phone at (785) 537-4995, (785) 537-0813 (fax) or email:


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