Kruger Oxidation Ditch Overview

Aerial View of Kruger Oxidation Ditches at Princeton WWTP

The actual oxidation ditch is an elongated closed loop that resembles a racetrack in configuration. The mixed liquor flowing in the two channels of the ditch is separated by a wall that runs down the central axis of the ditch and terminates at the beginning of the semi-circular ends of the ditch. Mechanical brush aerators (rotors) span the channels of the ditch, with the brush partially immersed in the mixed liquor. A steel and concrete bridge covers the rotors and provides an access walkway that also eliminates the spray of aerosols into the air. The rotors aerate the wastewater and provide the mixing which keeps the activated sludge in suspension.

The Princeton Wastewater Treatment Plant consists of a three - (3) stage Anaerobic Selector and two (2) oxidation ditches operating in the A/O or BioDenipho modes of Phased Isolation Ditch (PID) Technology. The equipment associated with the process includes three (3) 2.4 HP POP-I submersible mixers, two (2) 5.0 meter motor actuated influent weirs, four (4) 6.0 meter direct drive horizontal brush aerators (rotors) with 40 HP motors, four (4) 9.0 HP POP-I submersible mixers, two (2) 5.0 meter motor actuated effluent weirs, two (2) dissolved oxygen probes and monitoring systems and a PLC-based control panel/SCADA (Supervision, Control And Data Acquisition System) system.

The Anaerobic Selector (Influent Mixing Tanks) located in front of the ditches at the influent end, is divided into three stages. Each stage of the Anaerobic Selector contains a submerged mixer designed to maintain the suspended solids in solution while minimizing turbulence at the liquid surface. A gear-reduced, waterproof submerged motor drives the mixer.

Following screening and grit removal, influent wastewater enters the process through the Anaerobic Selector. At the same time, Return Activated Sludge (RAS) is returned to the Anaerobic Selector from the clarifiers. The partitions in the Anaerobic Selector of the A/O / BioDenipho process provide for the staging that is necessary to ensure completion of the anaerobic conditioning. A 2.4 HP POP-I submersible mixer is installed in each Anaerobic stage to maintain solids in suspension and provide complete mixing between the wastewater and the RAS. The partitions have relatively small openings that allow the mixed liquor to pass to the next stage, but minimize the hydraulic head loss and prevent excessive back-mixing. The mixed liquor flows through the selector and is directed into a distribution chamber. The influent weirs, which are controlled by a PLC-based control panel, control distribution of the flow between the two ditches. Flow is directed into either of the two ditches depending on the status of the process.

End View of Anaerobic Selector (Influent Mixing Tanks)

The facility includes state-of-the-art, Kruger computer control systems. The system monitors and controls most plant equipment as well as biosolids facilities and the Princeton South Lift Station located near the Toyota plant.

The SCADA system is a graphical presentation of the physical plant where the monitored components, such as pumps, motors, valves and measuring equipment are in focus. The SCADA system allows for effective optimisation of the process and energy consumption, better overview, quick transmission of alarms, flexible control and quick and easy reporting. The Kruger SCADA system has built-in process knowledge, logically designed systems,  with a uniform and well-arranged user interface as well as professional graphics with moving elements.

Kruger SCADA System View of Ditch Operations

POP-I Submersible Mixer

Direct Drive Horizontal Brush Aerator

Oxidation Ditch Operating Modes

While the operating modes of the ditches are covered in detail along with the theory of operation and process description of each mode in the "Kruger Ditch Operations" section of this web site, a brief description of the modes are presented below.

The basic conceptual plant operating modes of the Kruger Oxidation Ditches are A/O and BioDenipho Plant.
However, due to the extremely flexible nature of the Kruger PLC based control system, an infinite number of additional modified or indeed unique modes of operation can be programmed in to the system as required by the specific needs of the plant at any given time, and the Princeton plant is in fact currently operating in a modified mode of operation which is detailed in the following summary of the modes of operation.

A wastewater treatment plant employing the A/O and BioDenipho processes resembles a conventional oxidation ditch system. The major components common to both a conventional oxidation ditch and the A/O / BioDenipho processes are a closed loop reactor basin where aeration or oxygenation of the mixed liquor takes place, a clarifier for settling the mixed liquor, and a return sludge pumping system. The distinguishing features of the A/O / BioDenipho plant include the Anaerobic Selector prior to the oxidation ditches, the series flow pattern in the ditches, and the alternating process conditions (phasing) within the oxidation ditches.

When the process is operating in the BioDenipho mode, the mixed liquor passes through the oxidation ditches in series for the majority of the process. However, there are periods of time, or phases, when the mixed liquor only passes through one ditch, while the other ditch is essentially isolated from influent. The specific phases of the BioDenipho process is discussed in detail in the "Operations" pages of this section.

As the mixed liquor passes through the ditches, the process conditions within the ditches are alternated between oxic and anoxic to accomplish both nitrification and denitrification without internal recycle pumping. The hydraulic capacity or volume in the oxidation ditches is large in comparison to the influent flow volume.
Therefore, the concentrations of the pollutants are greatly diluted upon entering the ditch. This dilution of the influent wastewater helps the oxidation ditch process resist upsets from shock loadings of organics.

The Princeton Wastewater Treatment Plant is able to operate in the BioDenipho mode of operation up to approximately 2.3 MGD. At flows greater than 2.3 MOD, the oxygen requirements for the system will exceed that provided, and the system should then be operated as an “A/O Ditch”.

The distinguishing feature of an A/O plant is the division of the reactor basin into two zones: non-aerated sections referred to as the anaerobic zone (or "Anaerobic Selector,” as previously described), and aerated sections called the oxic zone, which comprises the remainder of the reactor basin. The two zones can be further subdivided into two or more equal stages to promote plug flow conditions. For Princeton, the oxic zone is one stage only and effluent from the Anaerobic Selector is evenly split between each of the two ditches where controlled nitrification and limited simultaneous denitrification will take place. There are only several differences between A/O operation and BioDenipho operation. First, influent flow is split evenly in the A/O process rather than directed into one ditch or another (BioDenipho), (i.e. the ditches will be operated in a parallel rather than in series). Also, no phasing of ditch operation will occur. Instead, each ditch will be operated under oxic conditions at all times.

The Princeton Wastewater Treatment Plant is currently not required to meet specific effluent total nitrogen limits or phosphorus limits. Utilizing the BioDenipho process would provide several beneficial results.
First, the BioDenipho process would allow the facility to recover a fraction of the alkalinity that is consumed during the nitrification process. As a result, the system would be more resistant to shock loadings and would be able to maintain a suitable pH for biological activity. Second, the BioDenipho process would allow the facility to reduce the daily operational costs by reducing the BOD loading that must be oxidized in the ditches. The process of denitrification, as discussed on the "Operations" pages, consumes influent BOD without oxygen and therefore reduces the demand on the aeration equipment. Third, utilizing an Anaerobic Selector inhibits the growth of filamentous organisms and insures optimum settling conditions in the secondary clarifiers. For these reasons, the Princeton plant should under ideal conditions take advantage of the benefits that can be achieved through controlled denitrification and biological phosphorus removal.

However, due to the nature of the influent waste received into the plant from the various manufacturing facilities within the Princeton system, the plant cannot operate in this mode, or indeed in the A/O mode either, as the plant could not remain within the discharge limits as specified in federal and local regulations if operated strictly in either of these modes.

Instead, the plant is operated in a mode referred to as "Extended Air Mode".
In simple terms, Extended Air Mode is a split between the A/O and BioDenipho modes where, as in the A/O mode, there is no phasing of ditch operation and each ditch is operated under oxic conditions at all times, and as in the BioDenipho mode, the ditches are operated in series.
This mode keeps the D.O. around 2.5 - 2.8 mgl. If the D.O. rises to 2.8, the PLC shuts the rotors off and when the D.O. decreases to 2.4 the PLC restarts the rotors. There is a 240 second delay (dead band) when the set points are reached to prevent the continuous recycling of the rotors.
This mode of operation, along with the others is explained in detail in the "Operations" section pages.

Illustration of Ditch Operations

The effluent weirs control the liquid level in the ditches, as well as the flow path through the ditches. The submergence of the rotor can be varied from 4 to 11 inches. The level sensor in the ditch monitors the liquid depth and feeds that data to the PLC. The PLC processes the data and controls the operation of the weirs. If the level in the ditch were increasing, the weir would be lowered to maintain the desired submergence. Similarly, if the level in the ditch decreases, the weir would be raised to account for the change. The intention behind this feature is to operate the rotors at their most efficient submergence to reduce power consumption and maximize oxygen transfer.

The weirs are also controlled by the PLC for process purposes. The flow path through the ditches varies with the different phases of the BioDenipho process, which will be discussed in detail in the Operations Section pages. Raising and lowering the effluent weirs in the two ditches controls these flow paths.