Oxidation Ditch...

Double Oxidation Ditch...

The Double Ditch (D-Ditch) mode of Phased Isolation Ditch (PID) technology is a completely self-contained treatment process. The D-Ditch Process consistently produces a fully nitrified effluent, low in BOD and suspended solids, without the use of clarifiers. D-Ditch plants are often compared to Sequential Batch Reactors (SBR) because they operate without an external clarifier. However, the D-Ditch Process offers several features that increase process stability and decrease operating complexity when compared to SBR's. These features include a constant water level and continuous discharge which lower the surface settling rate, lower the weir overflow rate, optimize the aeration unit process, and eliminate the periodic surges of effluent associated with SBR's.

The D-Ditch Process operating cycle consists of two main phases (A & C), and two intermediate phases (B & D). As shown in the operating cycle schematic Phases C & D are mirror images of Phases A & B. During Phase A, the influent is directed to the ditch that is under aeration (Ditch 1). The rotors aerate the mixed liquor, resulting in degradation of the influent BOD and nitrification of ammonia-nitrogen. The rotors are idle in Ditch 2, which is serving as a clarifier. The mixed liquor flows from Ditch 1 to Ditch 2 because the ditches are hydraulically interconnected by a port in the common wall, and the effluent weir in Ditch 2 is lowered to allow treated and clarified effluent to be discharged. During Phase B, the influent is redirected to discharge to Ditch 2, which continues to operate in the settling and discharge mode. The duration of Phase B is relatively short, and the volume of influent received during Phase B is small in comparison to the volume of Ditch 2. Therefore, the influent has little effect on the treated effluent being discharged. Ditch 1 is isolated. The rotors in Ditch 1 are idle in Phase B to allow the sludge blanket to drop in preparation for Phase C, when Ditch 1 will begin discharging effluent. Phase C is initiated by raising the effluent weir in ditch 2, and lowering the effluent weir in Ditch 1. The rotors are activated in Ditch 2, which continues to receive influent. The hydraulic gradient through the ditches has been reversed. Ditch 1 serves as a clarifier settling solids and discharging treated effluent. The D-Ditch can easily be expanded and upgraded by either adding another oxidation ditch and operating the Triple ditch Process, or by adding clarifiers and operating in the BioDenitro Process.

Triple Oxidation Ditch...

The Triple Ditch (T-Ditch) mode of PID technology is an expansion and upgrade of the double ditch process that efficiently performs controlled nitrogen removal without an internal recycle stream or external clarifiers. The T-Ditch Process, the three hydraulically-interconnected oxidation ditches alternatively serve as oxic, anoxic, or quiescent reactors, performing nitrification, denitrification, and clarification respectively. Because of the flow direction through the oxidation ditches is periodically reversed, RAS pumping, and internal recycle streams are not required. The T-Ditch Process can be designed for secondary treatment with full nitrification, or for nitrogen removal using the BioDenitro Process adapted to the T-Ditch layout. The BioDenitro Process is described in detail in the following section.

The T-Ditch is equipped with the simple influent and effluent hydraulic controls typical of all PID systems. During the main phases, influent is directed to one of the outer ditches forcing mixed liquor through the center ditch to the other outer ditch, which serves as a clarifier and discharges treated effluent. Influent flow diversion is controlled with three motor operated weirs. Each weir trough discharges to one of the ditches. Depending on the status of the process, one weir will be lowered while the other two weirs remain raised. The T-Ditch Process is the most energy efficient biological nitrogen removal process available today. As with the Double Ditch Plants, Triple Ditch plants can easily be expanded by adding clarifiers. Typically, clarifiers will increase the capacity of the plant by 50%, cost-effectively keeping pace with community growth.

Bio-Denitro Oxidation Ditch...

The BioDenitro mode of PID Technology is an energy-efficient wastewater treatment process that consistently achieves high levels of nitrogen removal without internal recycle pumping or chemical dosing. Since its' introduction in the 1976 US EPA Nitrogen Control manual, more than 100 BioDenitro plants have been constructed worldwide. The 3.0 MGD Ocoee, Florida Plant was the first of many US BioDenitro installations.

The BioDenitro Process was initially developed as an inexpensive method of treating wastewater in response to the 1970's energy crisis. As a result, denitrification was included in the operating scheme from the onset as a means of reducing the overall energy consumption. Oxidation ditches, including plants designed for nitrification, typically possess a considerable amount of excess volume. The BioDenitro Process utilizes this excess volume for controlled anoxic denitrification within the ditches in order to reduce the overall oxygen requirement. As a result, alkalinity lost in the nitrification process is regained, stabalizing process conditions. Because the BioDenitro Process does not require separate anoxic reactors for denitrification, it can easily be implemented at most existing oxidation ditch plants to reduce operating costs and increase effluent quality.

Phased Isolation Ditch systems, such as the BioDenitro Process, consist of interconnected oxidation ditches which are equipped with motor-operated influent and effluent controls, mixing equipment, and aeration equipment. The influent distributor and effluent weirs associated with a Phased Isolation Ditch provide complete control over the flow pattern in the BioDenitro process. For instance, in the main phases of the BioDenitro Process, the mixed liquor follows a series flow pattern through the ditches. However, the series flow pattern in Phase C is a mirror image of the series flow pattern in Phase A. The series flow pattern is initially produced by raising the effluent weir to impede the discharge of flow in the ditch receiving influent (Ditch 1). The effluent weir in the adjacent ditch (Ditch 2) is lowered to allow flow to discharge. Because the ditches are interconnected by an open port in the common wall, a hydraulic gradient is established that forces the mixed liquor from Ditch 1 to Ditch 2. Reversing the flow pattern is a simple operation that involves raising or lowering the effluent weirs, while redirecting the influent inflow via the influent distributor. In order to reverse the flow pattern, the influent distributor redirects the forward flow to enter Ditch 2 instead of ditch 1. Simultaneously, the effluent weir in Ditch 1 is lowered, while the effluent weir in Ditch 2 is raised to impede the discharge of effluent. This reversal of flow direction is automatically controlled via PLC's or timers and occurs approximately every two hours.

Method of Operation...

In most conventional treatment systems, nitrates are created in an oxic reactor and then transferred back to an anoxic reactor to facilitate denitrification. In the BioDenitro Process, the oxidation ditches alternate between oxic (nitrification) and anoxic (denitrification) phases. During oxic phases, influent ammonia-nitrogen (NH₃ - N) is oxidized to produce nitrate nitrogen (NO₃ - N). During anoxic phases, influent BOD serves as the carbon source required to reduce the nitrates to nitrogen gas, while submersible mixers maintain the biosolids is suspension. The BioDenitro Process operating cycle includes two main phases and two intermediate phases. During the main phases of the process (Phase A and C), one ditch will always operate in the anoxic (denitrification) mode while the other ditch operates in the oxic (nitrification) mode. During the shorter duration intermediate phases of the process (Phase B & D), both ditches operate in the oxic mode.

In Phase A (90 minutes), Ditch 1 is anoxic and nitrates produced during the previous three oxic phases in Ditch 1 are denitrified. Influent BOD serves as the carbon source required to fuel the denitrification, while submersible mixers maintain the biosolids in suspension. Ditch 2 is oxic, and discharges treated effluent to the clarifiers. In phase B (30 minutes), both ditches are oxic. The influent is directed to Ditch 2, which continues to discharge treated effluent to the clarifiers. In Phase B (30 minutes), both ditches are oxic. The influent is directed to Ditch 2, which continues to discharge treated effluent to the clarifiers. Ditch 1 is isolated to allow nitrification of the ammonia that entered the ditch during Phase A. In Phase C (90 minutes), the effluent weir in Ditch 2 is raised while the effluent weir in Ditch 1 is lowered. The rotors in Ditch 2 are deactivated, and the mixers are activated. Anoxic conditions are produced in Ditch 2 which causes nitrates, created during the previous 3 phases, to be reduced to nitrogen gas. Ditch 1 is oxic (nitrification) and is discharging treated effluent to the clarifiers. Phase D (30 minutes) is another intermediate phase similar to Phase B. At the end of this phase, the operating cycle will be complete and will be repeated, beginning with phase A. Typically 4 to 6 complete cycles will be performed over one hydraulic retention time. In addition, effluent is always discharged from an oxic ditch to minimize the ammonia concentration discharged from the plant.

1. The influent distributor is a motor-operated gate that changes position from left to right approximately every 2 hours. In the phase of the BioDenitro operating cycle (above), the distributor is directing the influent and RAS to Ditch 2. This supplies the required carbon source fro denitrification.
2. Low-speed submersible mixers impart the velocity to amintain the biosolids in suspension during the anoxic phases of the process. the mixers also supplement the rotors as needed in response to automatic DO control during oxic phases of the process.
3. The motor-operated effluent weir changes position from raised to lowered, or vice versa, approximately every 2 hours. In this phase of the process, the weir in Ditch 2 is raised.
4. Dentrified mixed liquor flows from Ditch 2 to Ditch 1 due to the hydraulic gradient set up by the influent distributor and effluent weir.
5. A probe monitors the Dissolved Oxygen concentration in the mixed liqour and transmits a signal to the control panel, which controls the operation of the rotors via a custom programmed PLC.
6. Brush aerators (rotors) maintain oxic conditions within the mixed liquor during nitrification phases of the process. The rotor bridge eliminates noise and aerosols, while providing access for equipment maintenance.
7. The effluent weir in Ditch 1 is lowered to allow the discharge of mixed liquor from the process. the discharging weir also controls the submergence of the rotors.

Bio-Denitro Process Flexibility...

One of the most unique aspects of the BioDenitro Process is flexibility. The volumes allocated for nitrification and denitrification can be increased or decreased by varying the length of the main and intermediate phases. Because of this flexibility, process operation can be adjusted to match the actual composition of the influent wastewater. Consider the operational strategy previously described for the BioDenitro Process. In Phases "A" and "C", one-half the volume is oxic and the other half is anoxic for 90 minutes. In Phases "B" and "D", the entire volume is oxic for 30 minutes. The amount of time over the 4-hour cycle (during which the entire volume is dedicated to oxic conditions) is 150 minutes of the total 240-minute cycle, or approximately 63% of the reactor volume is oxic. Similarly, the reactor volume is anoxic for approximately 37% of the time. By adjusting the phase lengths, the volumes allocated to nitrification and denitrification can be controlled.

For example, if we apply a duration of 60 minutes for the main phases, the oxic and the intermediate phases, the oxic volume is increased to 75%, and the anoxic volume is decreased to 25%. By this simple modification, the oxic volume has been increased by more than 19%. This is what is meant by a "time-based" treatment strategy. The operation of the BioDenitro Process can be customized to the daily, weekly, monthly, or seasonal variations experienced at a plant. During low summer flow periods, for example, the anoxic phases may be extended to maximize the denitrification, thus minimizing the aeration requirements as well as the associated energy costs. Conversely, during winter months, the oxic phases can be extended to ensure a sufficient supply of oxic sludge age in order to maintain complete nitrification. The combination of diurnal variations and an extensive design period results in a wider range of loadings that a wastewater treatment plant has to process. The oxygen demand exerted at a plant will vary with these influent loadings. Typically, aeration equipment is operated at a level high enough to insure that the peak demand is satisfied, producing periods of over aeration and wasted energy. By matching the oxygen supply to the demand, energy consumption will be significantly reduced and treatment performance will be enhanced. The BioDenitro Process incorporates separate aeration and mixing equipment into the standard plant layout, along with automatic Dissolved Oxygen (DO) control. In this manner, the operation of the aerators can be discontinued when an acceptable DO concentration is achieved in the mixed liquor. Submersible flow boosting mixers are simultaneously activated to maintain the biosolids in suspension. The resulting oxygen supply is thereby effectively paced to match the diurnal variations of the demand, regardless of whether the plant is processing 25% or 100% of the design capacity.

Energy efficiency And DO Control...

Each oxidation ditch was equipped by a DO probe, which continuously monitors the DO concentration in the mixed liquor and transmits the data to the Programmable-Logic Controller (PLC) in the control panel. The control panel includes an adjustable DO set point, with an associated dead band of + 0.25 mg / L. If the DO set point is set at 1.5 mg / L, for example, one rotor will run until the DO concentration reaches 1.75 mg / L. At that point, the operation of the rotor will be discontinued, and the mixers will be placed in operation. Conversely, when the DO concentration falls to 1.25 mg / L, the operation of the mixers will be discontinued, and a rotor will be placed in operation. If one rotor cannot maintain the DO concentration above 1.25 mg / L, a second rotor will also be placed in operation. The alternating sequence of the ditches between oxic and anoxic conditions, in response to the phase changes, can also be seen on the DO strip chart. During Phases "A" and "C," one ditch is oxic while the other ditch is anoxic. During Phases "B" and "D,' both ditches are oxic.

Bio-Denitro-Pho Oxidation Ditch...

The BioDenipho mode of PID technology is an expansion of the BioDenitro process, incorporating anaerobic selector technology to promote biological phosphorus removal from the wastewater. The BioDenipho Process combines the process flexibility and energy efficiency of the BioDenitro Process with the advantages offered by an anaerobic selector, resulting in a highly-efficient Biological Nutrient Removal (BNR) system.

Phosphorous Removal Mechanism...

The anaerobic selector separates activated sludge metabolism into two distinct steps: BOD uptake and BOD oxidation. By passing the Return Activated Sludge (RAS) and influent through the anaerobic selector, microorganisms capable of using stored polyphosphate as an energy source are proliferated. This energy is used to transport BOD into their cellular structure when free or combined forms of oxygen are not available for respiration. Typically, the anaerobic selector is a three- or four-stage reactor equipped with submersible mixers to maintain biosolids in suspension. RAS is discharged to the first stage of the selector, while raw influent is directed to the second stage. By staggering the RAS and raw wastewater influent locations, the volatile fatty acids and soluble BOD, which promote phosphorus-release, are not consumed during RAS denitrification. In the second stage, denitrified RAS is contacted with the influent wastewater in the absence of free or combined forms of oxygen. The anaerobic environment stresses the microorganisms, which begin to break down stored polyphosphate reserves into orthophosphate. Throughout the remainder of the anaerobic selector, orthophosphate is expelled from the microorganisms releasing energy that is used to absorb BOD into their cells. In subsequent anoxic and oxic phases, the BOD is oxidized and the cells reproduce. In the oxic phases, these new cells, along with old cells, replenish the phosphorus reserved within their cells. This results in a net phosphorus uptake. Phosphorus removal from the wastewater is, ultimately achieved by wasting phosphorus-rich sludge from the system. As an added benefit, the anaerobic selector inhibits the growth of filamentous bacteria that cause bulking sludge.

Anaerobic / Oxic (A / O) Oxidation Ditch...

The A/0 Ditch Process is the application of the A/0 Process in an oxidation ditch layout. The A/0 Process has been employed in full-scale operation since 1979, and consists of a staged anaerobic selector followed by multiple oxic reactors operating in series as shown in figure located below.



The anaerobic selector was initially developed to improve the settling characteristics of activated sludge by inhibiting the growth of filamentous bacteria, which cause bulking sludge. Filamentous bacteria are obligated aerobes passing the RAS through the anaerobic selector, which is devoid of free and/or combined forms of oxygen compounds; the filamentous bacteria are not able to proliferate. The anaerobic selector also promotes biological phosphorus removal from wastewater, which is described in detail in the description of the BioDenipho Process. The combination of minimal filamentous populations and an increased phosphorus concentration in the biosolids, produces a dense rapidly settling floc. As a result, effluent solids concentrations are reduced while the RAS concentration is increased. The increased RAS concentration enables the associated pumping rate to be reduced. In the A/0 Ditch Process, the staged anaerobic selector is followed by an oxidation ditch. Automatic DO control into the operating strategy of the A/0 Ditch Process maximizes simultaneous denitrification and minimizes energy consumption. Biological phosphorus removal eliminates the expense and excess sludge generated by chemical precipitation processes. In addition, the inhibited filamentous growth ensures consistent process performance during difficult treatment conditions, including low F:M ratios and low DO concentrations.

Another attractive feature of the A/0 Ditch Process is that upon expansion, nitrogen removal can be incorporated into the treatment scheme by adding anoxic tanks and operating in the A2/0 Ditch Process, or by adding a second ditch and controls to operate in the BioDenipho Process. Regardless of whether your plant's effluent limitations include phosphorus removal, or simply require 30/30 treatment, the advantages offered by the A/0 Ditch Process warrant your consideration.

Anaerobic - Anoxic - Oxic (A² / O) Oxidation Ditch...

The A²/0 Ditch Process is the application of the A²/0 Process in an oxidation ditch layout. The A²/0 Process is similar to the A/0 Process, however anoxic tanks are incorporated into the treatment scheme. The anoxic tanks are located after the anaerobic selector and prior to the oxic reactors. Nitrates are returned to the anoxic tanks from the oxic reactors via a pumped internal recycle line. In the A²/0 Ditch Process, a wall pump is applied to accomplish the internal recycle pumping. The wall pump is typically equipped with a Variable Frequency Drive (VFD) to provide the flexibility required for the pumping rate. Common wall design minimizes structural costs, as well as the plant footprint. The A²/0 Ditch Process can be applied to a single ditch, or double ditch layout. The 4.0 MGD Titusville, Florida WWTP design is similar to the layout mentioned before, but also incorporates second anoxic tanks into the treatment scheme in order to meet the stringent 5-5-3-1 (BOD-SS-TN-TP) effluent limitations. The combination of the high-rated A²/0 Process, with the "forgiving nature" of an oxidation ditch, makes the A²/0 Ditch Process a compact BNR treatment plant that is simple to operate and consistently produces a high- quality effluent.

Instrumentation and Process Control...

The Control Panel typically includes a Programmable Logic Controller (PLC) that is customized to meet individual customer requirements. From a basic configuration with a Keypad/Display Interface, the control panel can be expanded to include chart recorders, alarm annunciators, control switches, Push-buttons, indicators, and graphic displays. In addition, they can be made suitable for either indoor or outdoor installations. Instrumentation and control combine industry standard hardware components with the field-tested and refined Process and Control Algorithms to automate each of the Oxidation Ditch Technologies. In addition to monitoring and controlling Biological Process, the PLC control panel can be expanded to monitor and control other devices or subsystems located throughout the plant or collection system. All Oxidation Ditch Control Panels are SCADA ready which allows for a single cable connection to a PC- based Supervisory Control and Data Acquisition System. The SCADA system monitors and controls the process while storing relevant data that can be used to compute and report plant performance. With all the additional information a SCADA System provides, the operators gain valuable insight to the operation of the plant. As a result, operators become more confident in the plant's capabilities and are able to optimize the plant's performance.



The system can provide complete alarm monitoring, both local and via alphanumeric pagers. With this feature, the operations personnel will be notified immediately if there is an equipment problem or a process malfunction. This enables the end user to avoid significant emergency repair costs and headaches. With the pager option, man hours can also be reduced. Complete trending of all process variables is also included. The trends enable the operators to view the plant data at user-selectable time spans from 1 minute to 1 year, and provide the information needed to run the plant at peak efficiency. All the real-time information will be linked automatically to the manually entered laboratory data to create pre- formatted water quality and accountability reports. These reports can then be printed and sent to the State every month. SCADA System keeps track of run-times and the number of starts and stops for all devices. This information is logged and can be used for trouble-shooting or preventive maintenance. Even devices which are not monitored for run-times, such as flow meters, can trigger preventive maintenance alarms of accumulative time since last calibration.