Engineering Term Paper - Construction Dewatering Engineering

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Planning and authorization
The old wastewater treatment plant Duisburg - Kaßlerfeld that was built in 1954 had a mechanical - chemical stage of treatment consisting of screen, grit removal and settling tanks as well as a digester and a sludge deposit. Already in the early 1970s first plans were made for a mechanical - biological plant that could not be realised for several reasons. The plans included a sludge incineration plant, which got no approval since the air immission load was then already higher than allowed.

The new plan, which was submitted in 1982, contained a concept of biological purification that was designed for the breaking up of carbon. The biological treatment had to be altered in the cause of the process of approval of the concept in such a way that a nitrification of ammonia also became possible. By the act of approval of concept the plan to renew the wastewater treatment plant Duisburg-Kaßlerfeld was authorized on 1. September 1987.

The part of the plan concerning wastewater technology was revised after the authorization in order to allow a specific nitrification and denitrification as well as the elimination of phosphate. The revisions were included in the authorization by an alteration of the approval of concept in summer 1989.
Due to its importance for the infrastructural development in the catchment area the realization of the wastewater treatment plant Duisburg-Kaßlerfeld was subsidised by the federal government of Northrhine-Westphalia with a subvention covering up to 80 % of the costs of investment.
 

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Structural engineering and project management
The construction works started late in 1988. In summer 1992 the carcase work was completed. As a first measure the entire area was surrounded with a tightening wall in order to protect it against ground water floods. This wall is 60 cm thick, 1,450 m long and has a surface of about 36,000 m2. It reaches down to a maximal depth of 45 m in the underground. This complex construction was necessary because it was decided to avoid an intake pumping station for economical reasons. Thus, the wastewater treatment tanks lie below the level of the rest of the area.

The construction work was considerably influenced by the request to maintain the previous degree of wastewater treatment and the discharge of the wastewater during the entire time of construction. Due to the construction plans and the necessity to protect certain plants, which were already, or still in operation the construction work faced severe obligations. Additionally considerable masses had to be moved. In the allotment of the wastewater treatment plant alone about 500,000 m3 of soil had to be moved and about 52,000 m3 concrete had to be produced with 6,000 t of steel and 100,000 m3 of sheating.

The facts that all parts of the plant could be started up in time and that the estimated costs were not exceeded prove together with the now existing operating results the success of the project management and the quality of the means of control and supervision used.
 

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Wastewater treatment processes
The wastewater is treated mechanically, biologically and chemically. The mechanical treatment uses a screening plant, an anaerated grit removal tank and primary settling tanks. The wastewater runs through the screening plant in four parallel flows with a backraking coarse screen (60 mm clearance) and a fine screen (20 mm clearance) each. The screenings are dewatered and transported via converter belts to containers. The entire plant was erected in a closed building for reasons of possible emissions and in order to guarantee an undisturbed operation even during winter.

After flowing through the grit channels the wastewater enters the primary settling tanks. Due to the fact that alterations had to be made for nitrification, denitrification and phosphate elimination the total volume of the four primary tanks was reduced to 5,200 m3. This reduction effects a coarse sludge removal in only 0.6 hours.

The separation of the aerated sludge from the treated wastewater takes place in the final clarification for which there are five rectangular tanks. These final clarification tanks have a total volume of 40,000 m3. Two sludge scrapers each remove the sedimented sludge. Due to the fact that the primary tanks are very small the waste activated sludge is thickened seperately. The treated wastewater is discharged through perforated pipes.

The chemical treatment eliminates the phosphorous, which together with nitrogen can cause an undesired mass development of algae in the receiving waters.

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Operation and control
All information relevant for the operation and control of the plant are transmitted to the operation center. Here the information is processed with the help of a computer and thus provides the necessary data for the staff. With the help of colour screens the shift-leader gains detailed insight into the conditions of any part of the plant and can take the relevant measures in case of disturbances. A total control of the plant is guaranteed by a large mosaic plate which shows the entire plant and which immediately signals disturbances. Apart from the operation center there are several sub-centers where the power station, the dewatering of sludge, the sludge digestion and the sludge thickening are controlled.

In order to operate and control the plant according to the relevant standards and to optimise the costs of energy consumption complex plants for process control were installed. The process control is of special significance for the operation of the biological treatment.

Influx and discharge of the wastewater treatment plant as well as the internal wastewater and sludge flow are not only controlled by the local analytical instruments which are continuously working but, more important, by the laboratory of the wastewater treatment plant and the central laboratory of the Ruhrverband in Essen. The operational data are registered in reports and can be plotted as diagrams.
 

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Sludge treatment processes
The Sludge treatment processes consist of the unaerobic stabilization of sludge by digestion and the mechanical dewatering of sludge. This plant was started up in spring 1991.

The main part of the sludge stabilization plant consists of three digesters with a volume of 8,720 m3, each which are operated, in parallel as mesophilic reactors at a temperature of 35 to 38 oC. The organic substances of the sludge are decomposed in the digesters to such an extent that the sludge can be dewatered afterwards and disposed without the emission of odor. The sludge remains in the digesters for about 21 days. The digestion process produces about 10,000 m3 of gas per day, which is stored in two gas tanks with a volume of 4,000 m3 each and then used for the generation of energy and compressed air in gas motors. The waste heat is used in turn for providing the digestion tanks with the temperature needed. Additionally, the buildings of the wastewater treatment plant are heated with this waste heat.
Whereas the primary sludge can be pumped directly from the primary clarifiers to the digesters the waste activated sludge from the secondary tanks has to undergo a preliminary thickening. This can be affected statically in gravity thickeners as well as mechanically by centrifuges. Both processes can be operated either in series or in parallel.

The digested sludge is dewatered in three chamber filter presses with a volume of 14m3 each. In order to improve the conditions of dewatering the digested sludge is conditioned by adding slaked lime and ferric chloride. Additionally it is statically thickened in a reaction thickener for 24 hours. With the help of high-pressure pumps the conditioned sludge is pumped to the chamber filter presses. By dewatering the digested sludge is concentrated to 40% of dry solids. This means a reduction of volume of about 90%. The supernatant and the filtrate, which are thus obtained, are led to the influx of the wastewater treatment plant. After the pressing the chamber filter presses are opened and the dewatered sludge falls into the silos situated underneath the presses. From there lorries carry the dewatered sludge to the mono-deposit on the Ruhrinsel Raffelberg about seven kilometers away. (ruhrverband.de)
 

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Sludge Dewatering Equipment
When industrial or municipal wastewater is treated through settling or flotation, its liquids and solids are separated. The water is discharged, leaving the concentrated solids behind. Many types of equipment are available for sludge dewatering, including belt filter presses, sludge dryers, vacuum filters, centrifuges, screw presses, plate and frame presses and drying beds. The type of sludge generally determines the type of equipment required. Treatment chemicals often are used in conjunction with sludge dewatering equipment to improve solids capture and enhance equipment performance. Numerous choices are available today, from single dewatering components to complete customized dewatering systems. In many cases, PLC instrumentation also makes possible the control of an entire dewatering system from one location. (pollutionengineering.com

Project Scope
Selection of the proper equipment and compatibility with the existing processes is key to an effective dewatering program. The following is a brief discussion of the work tasks involved in addressing these key issues as well as others. There are a number of dewatering options, which can provide the solids concentration desired for dewatered biosolids storage and land application. However, of these options the belt filter press and the centrifuge are generally the most common and cost effective for the size of Sandy’s process. These two processes have little in common except for being capable of dewatering wastewater sludge. The technology, size of equipment, power consumption, chemical usage and end product all differ between the two processes.

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The existing solids handling facilities will need to be evaluated to determine their compatibility with the proposed process. Currently, the City uses a lime slurry system to raise the pH of the liquid sludge in order to stabilize it for land application. Maintaining this system would result in processing a lime-stabilized sludge through the dewatering equipment. There are concerns with using this lime system with any dewatering equipment options due to the abrasive characteristics of the lime slurry and the potential for it to foul equipment.

With the dewatering process, water is removed from the biosolids and reintroduced back into the plant’s waste stream. This filtrate is relatively high strength wastewater and, at the volumes released from the equipment during operation, it has the potential to upset the treatment process. Provisions may be necessary to convert a cell in the existing WASH tank to hold the filtrate and release it slowly back into the waste stream, similar to the existing decant process. With a dewatered biosolid, the process of transporting and storing biosolids on site will dramatically change. Once the sludge is dewatered, its characteristics are similar to soil. Conveyers and loaders are necessary to move the material and it will require covered storage areas so it can be piled and kept dry until it is applied to the land. In addition, transportation and land application equipment will need to be acquired to properly land apply the material. (ci.sandy.or.us)

Backflow prevention
A backflow preventor is a mechanical device used to prevent foreign materials from entering and contaminating the drinking water supply. Plant Engineering performs, in conjunction with the City of Newport News, an annual backflow prevention device inspection and test of all connections to buildings and water main supplies. Each device is tested to demon. Potable water is processed through a filtration system to remove ions for the LCW system. The Plant Engineering maintains the LCW and ICW systems Department. The systems provide a continuous source of LCW for cooling accelerator support systems during accelerator operations, as well as other water-cooled equipment. The ICW system, which removes heat from the LCW for the end station beam energy dissipaters, contains corrosion inhibitors to protect the piping and heat exchanger surfaces. The CW system removes heat from the ICW system or the LCW system in all areas except the end station beam energy dissipaters. The CW is processed and the heat is discharged to the air through evaporation.
 

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Fire suppression water
The potable water system provides the source for all built-in Jefferson Lab fire extinguishing systems. The Plant Engineering Department manages this system. Plant Engineering's Maintenance Management Program ensures that fire-fighting water pressures are adequate and that sprinkler systems are operable.

Cooling tower water
Cooling towers remove/reject unwanted heat from air conditioning, cryogenic, and other systems. Potable water for use in cooling towers is treated to prevent corrosion and inhibit bacterial growth. Note that once water is introduced as make-up water in the tower basin, it is no longer potable. Plant Engineering manages this program to support facility usage. Implementation of the Maintenance Management Program ensures cooling tower water quality.


Activated Water Management Program
Water that accumulates within the tunnels, beam energy dissipaters, and experimental halls may become radioactive as a result of high-energy photons and neutrons interacting with stable oxygen, deuterium, and impurity atoms in water. The Activated Water Management program provides containment for potentially contaminated water from any discharge so that it can be sampled and the total activity counted prior to being discharged. Water that is detected at levels above background values for radioactivity must be disposed of through the sanitary sewer distribution system or by other approved means. Water that is at or below background levels may be discharged to the surface water runoff system. (jlab.org)


 

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