In aqua press 3/2006 Austrian water expert Raimund Haberl gave an overview of the range of reliable low-cost technologies currently available to treat wastewater in rural areas. In addition to plant-covered retention soil filters, oxidation ponds, biofilm reactors and various hybrid reactor systems, he also mentioned aeration systems and so-called sequencing batch reactors (SBR). One of the latter type has been in operation in the village of Bechstedt- Wagd in Thuringia, Germany, since 2005.
The choice of this technology stems from the disparate land uses in Bechstedt-Wagd. The village, which is located merely ten kilometres from the city of Erfurt, has been a vital source of food supply to the city all throughout history. Even today, large horticultural gardens dominate the townscape. In recent years, the town has also become increasingly used as an accommodation centre for commuters working in the Thuringian capital. And so a new part of town has developed next to the old one.
Difficulties with outlet values
The resulting disparity of land use and infrastructure often caused problems in the treatment of domestic sewage generated by some 260 inhabitants living in town. Up to 2005, domestic sewage from the old town used to be collected via combined sewers in pits without an outlet as well as in partial and fully biological small wastewater treatment systems.
The allotment gardens and the new part of town were serviced by a separate sewer linked to a decentralised fixed-bed reactor with a capacity of roughly 250 population equivalents (PE). The large fluctuation of water flow, which resulted from the inflow of combined sewage as well as from the different seasonal use of the allotment gardens, repeatedly led to difficulties in system operation.
Moreover, due to its long retention time in the sewer system the effluents were often in an advanced state of decomposition upon arriving at the treatment plant. Subsequently, the mandatory limits at the outlet often failed to be observed. To tackle these problems, the local authorities of Bechstedt-Wagd decided to build a central small wastewater treatment plant with a total capacity of 600 PE, which was to take up sewage from all parts of town. The tender documents originally envisaged a classical two-tier aeration system.
In the end, however, the Sequencing Batch Reaktor (SBR) concept submitted by the German maunfacturer mall umweltsysteme won the contract. The new sewage treatment plant finally went into operation in October 2005, after a four-month construction period.
The Bechstedt-Wagd plant specifically is a single-tier fill-and-draw activated sludge system. After passing through a balancing tank, a screen and an aerated sand and grease trap, the wastewater enters an SB reactor where it is cleaned in batches. In contrast to continuous flow systems, where the processes of nitrification, COD load decomposition, biological phosphorus removal, denitrification and sedimentation are carried out simultaneously in separate tanks, in SBR technology these processes are carried out sequentially in the same tank, the so-called reactor.
The latter, a monolithic structure made of cast in-situ concrete, is open at the top. Reactor is multiply used At the start of a cleaning cycle, the reactor is half filled with wastewater derived from the 38 m3 balancing tank. Small oxygen bubbles are blown into the aerated sludge/wastewater mixture (fine bubble aeration) by a rotary blower and four diffuser plates.
An on-line measurement system monitors the set values of oxygen; the flow of incoming air is controlled by means of a frequency converter. In this phase, decomposition of organic carbon compounds and oxidation of ammonium compounds via nitrite to nitrate takes place, with microorganisms primarily taking up phosphorus.
Then fresh wastewater from the balancing tank flows into the reactor, filling it up to its nominal volume of 285 m3. During the subsequent aeration process, the ammonium compounds contained in the second fill are reconverted into nitrate as carbon is aerobically decomposed. When the biological treatment phase is completed, aeration is switched off.
The sludge is now left to settle at the bottom of the tank, while a layer of clean water forms at the top. A floating decanter draws clear water from the tank surface and transfers it to the effluent balancing tank; the floating sludge is retained. The excess sludge that has settled out at the bottom is pumped into the sludge silo.
Control system responds to fill level The fill level in the balancing tank is the central parameter that controls the treatment cycles of the SBR system. A new cycle will only start when a set level is reached. Until then the system remains in “idle” mode. During that period there is a short-term interchange between fill phases and idle/aeration phases to keep the biomass inside the reactor alive.
The wastewater treatment plant of Bechstedt- Wagd operates in fixed twelve-hour cycles and has variable exchange volumes. The minimum fill level of the SBR is fixed. When the sedimentation phase is completed, decanting takes place until a clearly defined level is reached. In dry weather flow the exchange volume is therefore often less than 5 %, while under combined sewage conditions it increases to 45 %. If the maximum fill level in the balancing tank is reached before the cycle ends, an emergency load is released into the SBR.
Advantages in terms of operation, cleaning and cost SBR technology has several advantages. One of them is that the cleaning process inside the reactor can be accurately adjusted to the hydraulic and biochemical situation. The balancing tanks which form an integral part of the overall plant system help to balance out flow fluctuations.
The manufacturer guarantees that outlet values remain below the limits indicated in the enclosed table. The process of sludge settling inside the reactor takes place undisturbed. This ensures a quicker and more complete sludge separation than in continuous flow systems. The use of diffuser plates reduces energy consumption by up to 30 % compared to other aeration systems. Diffusers can be individually shut off by ball valves and disassembled via baffles even when the tank is filled. This ensures uninterrupted plant operation in case individual diffusers break down.
Peter Gottschlich, who works with the plant operator Wasser- und Abwasserzweckverband Arnstadt und Umgebung, confirms the positive image of the system. The expert also praises the quality of the concrete and the monolithic design. Another clear advantage of the Bechstedt- Wagd SBR reactor is its low investment and economical operation – aspects much appreciated by the local authorities.
(Source: aqua press Int. 1/2007, Helmuth Ziegler)
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