Efficient drinking water treatment
To satisfy the minimisation requirements of the German Drinking Water Ordinance, water should be treated using as few chemicals as possible. The waterworks of Stadtwerke Arnsberg in the Sauerland region of Germany do just that for their supply region. Therefore, a modern ultrafiltration system (UF) is installed. To ensure that the extremely fine UF diaphragm pores do not become blocked, they are backflushed on a regular basis. By turns, alkaline or acidic cleaning agent as well as a disinfectant is metered to the backwash water. Efficient standard metering syst ems, equipped with solenoid and motor driven diaphragm metering pumps aid the treatment of potable water by Stadtwerke Arnsberg. Benefits: the use of chemicals is also lower thanks to precise metering.
Waterwork uses metering systems with membrane metering pumps
Stadtwerke Arnsberg in the Sauerland region of Germany supplies around 18,500 homes with 10.5 million litres of potable water every day through, a 450 kilometre long pipework system. The necessary water extraction and treatment systems were built at the start of the 1970s. In the event of extreme contamination of the surface water, as shown by the perfluorinated surfactants scandal in 2006 and the heavy rain and flooding incidents in 2007, the existing works could no longer meet the current requirements. To ensure the long term supply of good quality potable water, even in extreme conditions, the Mohnebogen and Langel waterworks were modernised. Investments in cutting-edge technology were made in both plants. The heart of the new system technology is a modern ultrafiltration system which also removes viruses and bacteria in the treatment process. In the Stadtwerke's largest water extraction and treatment system, in Mohnebogen waterworks, up to 600 m'/h of the best potable water can be produced. If one of the three other works breaks down, the supply region can sill be supplied.
The raw water is extracted using a slotted extract ion pipe, laid about 80 cm beneath the Mohne riverbed. It runs via a sedimentation chamber into a storage tank. In the tank, the turbidity, conductivity and pH value are measured. Two submersible pumps deliver the water for artificial groundwater recharging into four rectangular 15 x 85 metre large recharge basins. The bottom of the basins are covered with about 60 cm of filtered silica sand, which is used for slow filtration. The entire filtering area of the recharge basins is ap prox. 5,100 m2 - Using 21 pumping wells, an average 340 m3/h of enriched groundwater is extracted and collected in two wells. A maximum of 600m3/h can be extracted. Each collection pump is equipped with three pumps that deliver the waterworks for deacidification.
As soon as the flocculant has been metered to the water, a series of very complex chemical and physical processes starts. So that the flocculant has sufficient time to react, the water flows through disruptive elements / static mixing elements and reacts in a dwell tank. Large flakes form and bind the contamination contained in the water and also remove dissolved organic substances from the water. After a complete reaction, the flakes are separated on the ultrafiltration diaphragm.
The raw water is extracted using a slotted extract ion pipe, laid about 80 cm beneath the Mohne riverbed. It runs via a sedimentation chamber into a storage tank. In the tank, the turbidity, conductivity and pH value are measured. Two submersible pumps deliver the water for artificial groundwater recharging into four rectangular 15 x 85 metre large recharge basins. The bottom of the basins are covered with about 60 cm of filtered silica sand, which is used for slow filtration. The entire filtering area of the recharge basins is ap prox. 5,100 m2 - Using 21 pumping wells, an average 340 m3/h of enriched groundwater is extracted and collected in two wells. A maximum of 600m3/h can be extracted. Each collection pump is equipped with three pumps that deliver the waterworks for deacidification.
Deacidification
Deacidification is performed in four pressure filters that are filled with dolomitic filter material. If the pH value is shifted to the alkaline range, "acidic" raw water is diverted in a by pass to the deacidification process and blended with the deacidified wat er. Twice a w eek, the filters are automatically backflushed. The liquid level in the sedimentation tank is requested by a program and then flushing start s. A settling time of 48 hours is recorded and once this time has passed, a drain plug automatically opens. The turbidity of the deacidified water is measured. The water then flows into an interim tank. This enables it to be visually checked as well using an inspection glass.Volume-proportional flocculation
The intermediate pumping station pumps the water to the core of the new treatment plant, the four-line ultrafiltration block. On the inlet side, the flocculant is metered by a metering station: The metering station is fitted with four solenoid diaphragm metering pumps. They meter the flocculant at up to 111/ h at a pressure of about 7 bar. A gauge also records the metering of each pump. In combination with the metering pump, the flocculant quantity is continuously metered, precisely tailored to the water volume being treat ed. Proportional to the volume and dependent on the turbidity, this means that only the necessary amount of flocculant is metered. The pumps are connected to the central control system via a Profibus.As soon as the flocculant has been metered to the water, a series of very complex chemical and physical processes starts. So that the flocculant has sufficient time to react, the water flows through disruptive elements / static mixing elements and reacts in a dwell tank. Large flakes form and bind the contamination contained in the water and also remove dissolved organic substances from the water. After a complete reaction, the flakes are separated on the ultrafiltration diaphragm.
Wafer-thin pores remove the finest particles and microorganisms
The heart of the water treatment system in Mohnebogen waterworks is the ultrafiltration system with a total filtering area of 7,200m2. It removes all particulate substances contained in the water. The diaphragm of the four line UF system filters microbiological impurities itself, such as bacteria, viruses or parasites: thanks to its pore size of just 0,02 m - compared to a hair which is 80- 120m thick - even the finest particles are removed from the water.Metering prevents blocking
The filtered out residues and impurities are dissolved by regularly back flushing the UF modules. They are diverted as sludge wat er to the sedimentation tank and sedimented. The UF modules are also chemically purified: alternating with an acid and a base. This prevents the extremely fine pored diaphragms being blocked. Various metering stations ensure the correct volume of chemicals. One station is used to meter acid, another for adding the base. To absolutely reliably exclude bacterial contamination, the diaphragms are also treated with disinfectant every 30 days. The quantities of biocide required for this are metered by a third metering station.
The metering stations are set up in separate rooms, in which the associated tank storage with the chemicals is located. At the customer's request, this can be designed as a double decker tank storage: the storage tank is set in a steel frame. Another container can be placed on top of this to pour its contents into the low er tank as soon as this is nearly empty. The top, empty container can then be replaced by a full one very easily.
Standardized metering stations
The main components are virtually identical on the ready-assembled metering stations: each one has two motor driven metering pumps, one solenoid metering pump, relief and shut -off valves, flow meters, sensors and pulsation dampers.The metering stations are set up in separate rooms, in which the associated tank storage with the chemicals is located. At the customer's request, this can be designed as a double decker tank storage: the storage tank is set in a steel frame. Another container can be placed on top of this to pour its contents into the low er tank as soon as this is nearly empty. The top, empty container can then be replaced by a full one very easily.
Neutralisation
Before the water that accumulates when cleaning the UF modules can be fed back into the Mohne river, the conductivity and pH value is measured. The measurement decides whether rinsing water is transferred to the sedimentation tank or to the neutralization tanks. The neutralization tanks are used to collect and neutralize the chemically treated backwash water. In these tanks, the pH value is adjusted using a metering station with alkaline solution and acid and at the same time, chlorine is bound to sodium bisulfite. The process is monitored by means of continuous pH value measurement (for acid/base rinsing) and an excess chlorine measurement (for disinfection rinsing).Active Carbon Filter
After circulating the UF modules, the water flows into the new active carbon filter system. This consists of four pressure t anks. In norm al operation, at a fi lt ering speed of 8 m/h and a contact time of 23 minutes, about 40 0 m 3/ h of water can be treat ed. All pefluorinated surfactants and organic contamination are removed. In normal operation the active carbon filter is charged in series as two UF lines travel via two filters.
Before the treated water reaches the potable water tank , it is irradiated with ultraviolet light and thereby disinfected. The UV system is made redundant. One system can disinfect up to 580 m 3/ h water without chemicals. Two UV radiation sources are installed for the maximum treatment capacity of 600 m 3/ h. Ultrafiltration and irradiating water in the UV system means that it is largely unnecessary to add any chlorine.
Before the treated water reaches the potable water tank , it is irradiated with ultraviolet light and thereby disinfected. The UV system is made redundant. One system can disinfect up to 580 m 3/ h water without chemicals. Two UV radiation sources are installed for the maximum treatment capacity of 600 m 3/ h. Ultrafiltration and irradiating water in the UV system means that it is largely unnecessary to add any chlorine.
Emergency chlorination
Despite all the specified water treatment steps, if there is still microbiological bacterial contamination, then chlorine can also be added to the potable water. There is a special metering station available for this: equipped with a diaphragm metering pump, disinfectant can be easily, flexibly and precisely metered in quantities of 0.02- 11.3 l/h. The Stadtwerke use sodium hypochlorite for emergency disinfection, a disinfect ant perm it t ed in accordance with the DVGW (German Techn ical and Scientific Association for Gas and Water) Technical Rule 291 and which can be disposed of without harming the environment. The measurement device monitors the metering and accurately records the flow rate of the pulsalting flow. In combination with the metering pump, the metered volume is continuously metered, precisely tailored to the water volume being disinf ected.Summary
All metering systems with solenoid and motor driven diaphragm metering pumps meet the exacting quality requirements. As a result of the precise, volume dependent metering, the chemical consumption is lower than estimated. The seam less operation of the components and metering pumps used guarantees outstanding process reliability and effective cleaning and disinfection of the ultrafiltration diaphragms. Each treatment plant in the Mohnebogen and Langel works now supplies up to 600 m3/ h of perfect potable water.