Oxidation Ditch Technology: Principles and Applications
In 1954 the Netherlands built the world's first oxidation-ditch sewage-treatment plant. Its prototype was a ring-shaped racetrack-style intermittent reactor with sloped walls, used as an aeration tank by day and a settling tank by night, achieving BOD removal up to 97%. Its simple structure and good performance attracted wide international interest.
In an oxidation ditch (Oxidation Ditch), the whole sewage-treatment process — influent, aeration, settling, sludge stabilization and effluent — is completed within the ditch. The earliest ditches needed no separate primary or secondary clarifiers or sludge-return equipment. As scale and scope grew, it typically adopted extended aeration with continuous feed and discharge; the generated biological sludge is stabilized during aeration, needing no primary clarifier or sludge digester, greatly simplifying facilities. Not only environmental agencies but also the WHO attach great importance to it. Hundreds of plants exist in the USA and over a thousand in Europe. In China, oxidation-ditch research and practice began in the 1970s, and with its economic and simple advantages it has become the preferred process for small- and medium-sized municipal plants.
The Pasveer ditch then treated village/town sewage serving only 340 people. It was an intermittent-flow plant merging the four main contents of a conventional system into one ditch: daytime feed and aeration, nighttime settling, with BOD5 removal around 97%.
Using horizontal surface aerators for aeration and propulsion, the Pasveer ditch's aerator had to stop periodically to let sludge settle and discharge treated effluent. First-generation ditches were 1–2.5 m deep; to achieve continuous operation, various forms with secondary clarifiers were developed. This stage was mainly extended-aeration systems.
Thanks to its simplicity and easy operation, since the 1960s the number and scale of oxidation ditches have kept growing; treatment capacity has gone from 300 population equivalents to 10 million today. Treated subjects expanded from domestic to both municipal and industrial wastewater, with many industrial-wastewater projects adopting the technology.
New-generation ditches, using 1 m-diameter brush aerators (Mammoth Rotor by Passavant) and vertical aerators (DHV), gradually increased in depth. The Mammoth Rotor allows depth 3.5 m and width 20 m. The vertical-aerator ditch, later called the Carrousel ditch, reaches 4.5 m depth. This stage considered nitrification and denitrification (Simultaneous Nitrification/Denitrification).
In the early 1980s the USA first proposed the integrated oxidation-ditch concept with the secondary clarifier set directly inside the ditch. Within just over a decade the concept developed and applied rapidly, showing broad prospects. An integrated ditch fully uses the ditch's large volume and surface, modifying part of its structure or installing devices so separation occurs inside the ditch without disrupting normal operation. The US EPA termed this Innovative/Alternative (I/A) technology.
It is worth noting that the technologies and standards in this field are also continuously developing and improving.
This stage further considered using the ditch for phosphorus and nitrogen removal; many new concepts and design methods emerged. Besides extended-aeration low-load systems, there appeared 'high-load ditches', 'nitrification-required ditches', 'nitrification–denitrification-and-phosphorus-removal ditches' and 'sludge-stabilization-required ditches', plus many new ditch forms.
• DHV's Carrousel 2000, Carrousel Denit, and DHV–EIMCO Carrousel oxidation ditches
In addition, from an industry-development perspective, market demand is also driving technological progress.
(2) Boat-shaped integrated oxidation ditch
Besides this, many factors must be considered in practical engineering applications.
Per the US EPA's 2000 design guidance, the average ditch velocity should reach 0.25–0.35 m/s to keep activated sludge suspended. There are also power parameters.
Since the Pasveer ditch appeared in 1954, the oxidation ditch has developed through its simple way of treating sewage. After years of use it endures and has scored many breakthroughs: the Carrousel ditch in 1968, the Orbal ditch in 1970, the Carrousel 2000 in 1993, the Carrousel 1000 in 1998, and still developing, the Carrousel 3000 in 1999, plus the integrated ditch in the early 1980s.
The reason: the strength of oxidation-ditch development lies in its circulation, which is the intrinsic cause of its endurance; extrinsic causes are its multifunctionality, sludge stabilization, good effluent and easy management. Its main feature distinguishing it from other activated-sludge processes is the ring-shaped tank, or rather: as long as the channel is end-to-end connected and water circulates, the chosen design parameters, ditch type and operation bring great convenience to operators and designers, with very strong flexibility and adaptability and broad room for further research, development and application.
The oxidation ditch, also called oxidation channel, is named for its closed ring-shaped channel structure. It is a variant of the activated-sludge process. Because sewage and sludge circulate continuously in the aeration channel, it is also called 'circulating aeration tank' or 'endless aeration tank'. With long HRT and low organic load, it is essentially an extended-aeration system. The following are the main design parameters of the general method:
In addition, from an industry-development perspective, market demand is also driving technological progress.
The oxidation ditch uses a continuous loop reactor (CLR) as the biological reactor; mixed liquor continuously circulates in a closed aeration channel. It is usually used under extended aeration. It uses a direction-controlled aeration and agitation device that imparts horizontal velocity to materials in the reactor, so the agitated liquid circulates in the closed channel.
An oxidation ditch generally consists of the ditch body, aeration equipment, inlet/outlet devices, and guiding and mixing equipment. The ditch body's plan is usually ring-shaped, but can also be rectangular, L-shaped, circular or other; the cross-section is mostly rectangular or trapezoidal.
Because the oxidation-ditch method has long HRT, low organic load and long SRT, versus conventional activated sludge it can omit the equalization tank, primary clarifier and sludge digester, and some can omit the secondary clarifier. It ensures good performance mainly because it cleverly combines the CLR form with specific positioning of aeration devices, giving it unique hydraulic and operating characteristics:
(1) The ditch combines plug-flow and complete-mix features, helping overcome short-circuiting and raise buffering. Influent is usually placed upstream of the aeration zone and effluent further upstream. Influent is well mixed and dispersed through the aeration zone in circulation, and mixed liquor keeps circulating around the CLR. Thus the ditch is plug-flow in the short term (one cycle) and mixed in the long term (many cycles). This combination basically prevents short-circuiting by ensuring influent experiences at least one cycle, while providing large dilution and buffering. To prevent sludge deposition, sufficient velocity (generally >0.3 m/s average) is required, yet HRT is long, so large circulation flow (several to tens of times influent) is needed; incoming sewage is immediately diluted by large recycle, giving the system strong shock-load resistance and good treatment of refractory organics.
(2) The ditch has an obvious dissolved-oxygen gradient, especially suited to nitrification–denitrification. Overall it is completely mixed, yet liquid advances plug-flow; its aeration devices are fixed, so DO is high upstream in the aeration zone and gradually falls along the ditch length, with a clear gradient and low, essentially anoxic DO downstream. The ditch can be designed with aerobic and anoxic zones for nitrification–denitrification, not only using oxygen in nitrate to meet some demand but also replenishing alkalinity consumed in nitrification via denitrification. This helps save energy and reduce or even eliminate chemicals dosed in nitrification.
(3) The uneven power-density distribution in the ditch favors oxygen transfer, liquid mixing and sludge flocculation. Conventional aeration power density is only 20–30 W/m³ with average velocity gradient G > 100 s⁻¹. This favors oxygen transfer and liquid mixing and fully cuts flocculated sludge particles. When mixed liquor passes the calm transport zone to the late aerobic zone, average G < 30 s⁻¹, sludge still has re-flocculation opportunity, improving flocculation.
(4) The ditch's overall power density is low, saving energy. Once mixed liquor is accelerated to the ditch's average velocity, maintaining circulation only needs to overcome friction and bend head loss, so the ditch maintains flow and sludge suspension at much lower overall power density than other systems. Foreign reports say oxidation ditches cut energy use 20%–30% versus conventional activated sludge.
Also, domestic and foreign statistics show that versus other biological methods, the oxidation ditch features simple process, easy operation, good effluent, strong reliability, low capital cost and low operating cost.
Conventional ditch denitrification mainly uses the uneven DO distribution, designing alternating aerobic and anoxic zones for nitrogen removal. Its biggest advantage is removing organics and total nitrogen in one ditch without external carbon, so it is very economical. But the volumes and DO of aerobic/anoxic zones in one ditch are hard to control precisely, so nitrogen removal is limited and phosphorus removal almost nil. Also, in a conventional single-ditch, microbes often swing through aerobic–anoxic–aerobic environments, so nitrifiers and denitrifiers are not always optimal, affecting per-volume capacity.
Despite advantages of good effluent, strong shock resistance, high phosphorus/nitrogen removal, easy sludge stabilization, low energy and easy automation, a series of problems remain in actual operation.
When wastewater has much carbohydrate, unbalanced N/P, low pH, high sludge load, insufficient DO or poor sludge withdrawal, filamentous bulking is apt to occur; non-filamentous bulking mainly occurs at low temperature with high sludge load. High microbial load makes bacteria absorb much nutrient; at low temperature metabolism is slow, accumulating large viscous polysaccharides, greatly raising bound water on sludge, giving high SVI and bulking.
For bulking causes, different measures apply: from anoxia or high temperature, increase aeration or reduce influent to lighten load, or lower MLSS (control return) to reduce oxygen demand; if sludge load is too high, raise MLSS to adjust load, and if necessary stop influent and idle-aerate for a while; add nitrogen and phosphorus fertilizer to balance nutrients (BOD5:N:P = 100:5:1); for low pH add lime; bleaching powder or liquid chlorine (0.3%–0.6% of dry sludge) inhibits filamentous growth and controls bound-water bulking [11].
Large influent grease that the system cannot fully remove enriches in sludge and, with brush aeration, produces much foam; long sludge age and aged sludge also easily foam. Remove foam by surface spray water or defoamer — common defoamers are machine oil, kerosene and silicone oil at 0.5–1.5 mg/L. Increasing sludge concentration or moderately reducing aeration also controls foam. With much surfactant, pre-remove by foam separation or other methods; adding an oil-removal unit is also an option. Most important is strengthening source management to reduce high-oil and other toxic wastewater inflow.
Excess influent oil lightens the whole system's sludge; if its residence in the secondary clarifier is not well controlled, anoxia and septic sludge float-up occur; over-long aeration causes intense nitrification and high nitrate, leading to denitrification and nitrogen gas in the clarifier, floating sludge; also excess oil may carry sludge up.
On sludge float-up, suspend influent, break up or remove sludge, find the cause and adjust operation. For poor settlability, add coagulant or inert matter to improve it; for high influent load reduce influent or increase return; for fine sludge particles lower aerator speed; for denitrification reduce aeration and increase return or sludge withdrawal; for septic sludge increase aeration, remove accumulated sludge and improve hydraulic conditions.
In the ditch, for its unique mixing and treatment effect, mixed liquor must circulate at a certain velocity. The minimum is generally 0.15 m/s, and the non-depositing average should reach 0.3–0.5 m/s. Aeration equipment is usually brush and disc aerators; brush immersion is 250–300 mm, disc 480–530 mm. Versus ditch depth (3.0–3.6 m), the brush occupies only 1/10–1/12 and the disc 1/6–1/7 of depth, so upper velocity is large (about 0.8–1.2 m or more) while bottom velocity is tiny (especially below 2/3–3/4 depth, almost no flow), causing heavy bottom sludge accumulation (sometimes 1.0 m thick), greatly reducing effective volume and treatment effect and worsening effluent.
In addition, placing submersible thrusters upstream of the aerator actively promotes circulation in the brush's low-velocity bottom zone, solving low bottom velocity and sludge deposition. Dedicated thrusters make operation more flexible, of great significance for saving energy and raising efficiency.
(6) It has no treatment capacity at all for water with very small BOD.
