D.V. Goryainov, A.C. Dumtsev
The use of modern equipment for combined purification of wastewater by removing solid fragments or abrasive impurities is considered. A brief description of the design of the device is given outlining its characteristics and advantages. Experiments on the implementation in urban wastewater treatment plants are examined.
When running treatment facilities, problems associated with preliminary poor mechanical purification of wastewater from coarse impurities, namely, solid fragments and abrasive impurities (sand) arise. It should be noted that floating waste can carry up to 25% of the total amount of sand that is released during subsequent mixing and aeration. This sand sediment may pollute the tank structure and lead to disruption of the technological process. In addition, modern approaches to the design of purification facilities are being done without the inclusion of sedimentation tanks in the technological chain due to the fact that these tanks were responsible for holding a significant amount of solid fragments and sand. In this regard, the use of meshes and sand traps as the only structures for trapping coarse impurities is gaining popularity.
The RUSMET research and production enterprise conducts research work to improve the wastewater purification processes of living areas and industrial enterprises. With this aim, specialists in NPO RUSMET developed a series of compact combined plants for mechanical wastewater purification, RUMCOM series with a productivity of 10 to 300 l/sec and designed to solve issues such as separation, transportation, washing and squeezing of waste, sand and grease.
The effectiveness of the RUMCOM mechanical treatment plants was confirmed at the city sewage treatment plants in the city of Mozhga in the Udmurt Republic with a design capacity of 8000 m3 / day, the composition of which is formed by the city’s domestic and industrial wastewater.
The purpose of the first stage of the reconstruction of treatment facilities was the commissioning of a mechanical cleaning unit in 2018. To make this possible, NPO RUSMET developed, manufactured and installed a complex consisting of 2 combined RUMCOM-350 units with a capacity of 350 m3 / h each (one of which is supposed to be operational and the other a reserve). Each installation consists of mechanized grills, aerated sand trap and grease trap.
While preparing for the designing process, it was determined that due to the limited area of the room (100 m2) allocated for the complex it would be impossible to prepare each installation of the RUMCOM-350 in the building and carry out its installation in the same space. To solve this problem, it was decided that the designers would constructively develop the equipment in a modular design which would allow assembling of the unit from relatively smaller modules directly in the building.
The effluent from the sewage pumping station is fed into the pressure damping chamber via pressure pipelines installed in the building of the mechanical cleaning unit and flows out with the help of gravitational force into a box with a lower open sieve of the grills and goes through holes with a diameter of 5 mm. The screw grill is designed for effective separation, transportation, washing and squeezing of the waste. The grill consists of a sieve, a screw conveyor and a module for waste washing and squeezing. Solid particles are trapped on the inner surface of the sieve creating an additional filter layer. After some time, the sieve is cleaned with brushes installed around the perimeter of the screw and the waste is pumped up to where it is unloaded at a convenient level in a conveyor and then transported to a container. Brushes are washed using spray nozzles with a strong impact caused by the water. Near the discharge neck, washing and squeezing zone for the waste is designed, which provides an additional reduction in humidity and dries up the trapped solid particles by up to 30...40%. The built-in waste flushing system which serves to further optimizing of the cleaning process improves the ratio of carbon and nitrogen in the effluent and can significantly save money on waste disposal. When washing, soluble substances are separated from solid contaminants, as a result of which faecal matter is almost completely washed out and the weight of waste is significantly reduced (by almost 50%).
Further, the wastewater containing sand, solid inclusions of fine fragments (less than 5 mm), suspended organic matter and greases/oils floating on the surface flows into a horizontal aerated sand trap reservoir, in which sediments settle below the sand inclusive of other heavy solids which sink due to gravitational force. It should be noted that one of the problems in the sand traps is the collection and disposal of the precipitated sediments. The horizontal sand trap, generally, is equipped with mechanical scrapers with a chain drive. Such a system for sludge removal is extremely unreliable and the most vulnerable link in the chain drive of the scrapers. In the RUMCOM system, this function is implemented using a non-axial screw conveyor rotating in a bottom tray with a lining made of a wear-resistant polymer which moves the sand to the discharge zone where it is intercepted and brought up by an inclined conveyor. The sand trap reservoir is equipped with an aeration system which consists of separate modules that are easily removable (this simplifies cleaning) and is designed in such a way that a longitudinal row of pop-up air bubbles turns the contents of the reservoir in a spiral. As a result of this process, a more continuous “path” of sedimented suspended particles (not along a linear trajectory, but along a spiral) is created, which makes it possible to reduce the estimated reservoir length necessary for the deposition process. In addition, it became possible to adjust the operating parameters of the deposition system by simply adjusting the air supply (by taps on the airway). The aeration system also contributes to the sand washing, binding and floatation of oils and greases. Pop-up air bubbles stick to their molecules and raise them to the surface forming a foam which is collected using a scraper mechanism and is fed to the outlet pipe through a funnel for disposal or sent to the discharge screw of the grill.
At the outlet of the reservoir, a level control system using which you can adjust the working volume of the content is installed. Unloading shafts from the grills and sand traps of the 2nd plant are connected by transverse screw conveyors which allow transportation of trapped contaminants to the corresponding storage container. For hygienic purposes, to minimize vapours and odours, as well as for the convenience of flushing, prophylactic and routine maintenance, the upper plane of the tank is closed by light swing doors. All the equipment is made using 08Kh18N10 stainless steel and processed by shot blasting which improves the corrosion resistance of the product and creates an aesthetic matted surface structure. The welding seams were further processed by etching with a special solution.
The complex has low power consumption and produces an insignificant level of noise. The complex is managed by a local control system. Upon reaching the upstream level of the set value, the installation cycles are activated. The control system provides the logic of operation both in terms of the level of drains and in the cyclic settings specified by the PLC. The PLC provides reception and processing of signals from sensors and actuators, as well as communication with external systems via ModBusTCP, EthernetI, CANopen, etc.
After mechanical treatment, the wastewater is directed to the equalizer where the quantitative and qualitative equalization of the runoff composition necessary for the stable course of further water treatment processes takes place.
Findings
The modular design of the equipment made it possible to carry out installation and commissioning as soon as possible. The mechanical treatment complex required low investment costs, proved to be an economical and highly efficient system and ensured high-quality purification of water, increased reliability, and reduction in the required dimensions. It is also important to note that the maintenance of the equipment lowers the cost significantly because the system fully works in automatic mode.