Double benefits: improving ecology, saving
N.V. Tarasenkov, Candidate of Technical Sciences, Deputy Head of the Water Treatment Department of the Scientific and Production Company "Cortex"
The scientific and production company "Cortex" (St. Petersburg) has been purifying oil-contaminated wastewater at Russian Railways enterprises in a fairly economical way for the last 15 years. At the same time, it is possible to achieve a cleaner quality than the norms for drinking water.
Flotation plants have been installed at the treatment facilities of most Russian Railways enterprises, the designs of which were developed by VNIIZHT. Their total number on the road network is more than 800 installations. The design indicators for cleaning these flotation plants from oil contamination, incorporated into the design by the developer, are as follows:
- from 15,000 — 20,000 to 700 mg/l for the content of petroleum products in the treatment facilities of wagon repair depots;
- from 300-400 to 20 mg/l for petroleum products at locomotive depot treatment plants;
- from 5-10 to 2-3 mg/l for petroleum products during the purification of oil—contaminated stormwater.
Requirements of regulatory authorities to discharge standards for cleaning up oil pollution during discharge:
- 0.3 — 1 mg/l for petroleum products in urban sewage treatment plants;
- 0.05 — 0.1 mg/l, depending on the classification of the reservoir into natural sources.
As can be seen, the design indicators of water treatment significantly exceed the normalized indicators for wastewater discharge. Wastewater treatment can significantly improve these indicators and improve the environment. However, we should add that not all of them achieve the design cleaning performance.
In order to understand the complexity of wastewater treatment of railway enterprises, let us consider the nature and nature of oil pollution in water and the problems of cleaning them. Oil pollution in water can be roughly divided into three groups:
- floating petroleum products — large drops of petroleum products that float to the surface in 20-30 minutes. Such pollution is prevalent in storm oil-contaminated wastewater. Large drops of petroleum products fall out of the soiled soil of the depot area due to a leak in the sewer collector. Floating petroleum products make up the bulk of pollution in locomotive depots.;
- finely dispersed petroleum products — small droplets of petroleum products, usually crushed during pumping by centrifugal pumps. They float to the surface in sedimentation tanks in 6 to 8 hours, and are relatively easy to clean by coagulation—flotation treatment.;
- colloidal and dissolved petroleum products do not settle at all and are the most problematic for purification.
At the same time, the most difficult to clean is the wastewater from wagon repair depots. Consider the technology of oil products entering the water, for example, when washing bearings.
A powerful jet, organized by a centrifugal pump, washes away the grease. The polluted water is fed into a circulating water storage tank. Before changing the circulating oil-contaminated waters, this washing operation is performed repeatedly, while the oil-contaminated water is sucked into the pump at a relatively low speed.
When entering the pump, the linear velocity of the water increases hundreds of times, and the jet bursts with the formation of cavities (voids). At the outlet of the pump, the cavities "collapse", a pressure of up to 10 thousand kgf/ cm2 and a temperature of up to 10 thousand kgf/cm2 are formed. C is in the center of the collapse.
Micro-explosions destroy large and small droplets of petroleum products to a colloidal state, the most difficult to clean. At the same time, the concentration of recycled oil—contaminated water before its replacement reaches 15,000 — 20,000 mg/l for petroleum products, which is 40-50 times higher than the concentration in wastewater from locomotive depots.
Note that the phenomenon of "cavitation" in centrifugal pumps, in addition to the destruction of the pump impeller, also benefits from the production of ultra—stable (non-delaminated) emulsions. However, ultra—stable emulsions are bad for wastewater.
It is also important that, although it is polluted, it is water with a concentration 15 times lower than the minimum for disposal in urban landfills. However, for discharge into the sewer, it remains a volatile mixture even after cleaning on a flotation device.
The design 700 mg/l is 14,000 times higher than what can be discharged into open reservoirs and about 1,000 times into the urban collector.
Many organizations have tried to develop wastewater after-treatment after the flotation device themselves. Let's look at some of the options that are implemented in various depots.:
- aftertreatment using wood shavings (or sawdust). The concentration of petroleum products almost does not decrease, but secondary pollutants are formed in the form of phenols, which are normalized in the amount of 0.001 mg/l.;
- aftertreatment on membrane filters (ultrafiltration). The membranes are clogged with suspended solids (clay, sand) and fail after 0.5 — 1 h.;
- sorption aftertreatment using expanded clay, vermiculite is inefficient and non—technological;
- aftertreatment using activated carbons. The method is effective, but so expensive that the operating costs for a year will exceed the cost of treatment facilities.
As a result, all of the above methods were dismantled after implementation and short work.
To effectively address this issue, the specialists of the Scientific and Production Company "Cortex" have developed a method for post-treatment of wastewater from oil pollution. In it, the oil pollution of the depot wastewater itself is used as a filter material.
The figure shows a scheme for cleaning oil—contaminated waters, where the first stage is the CNII-5 flotation plant designed by JSC VNIIZHT, the second stage is a coalescing filter designed by NPF Cortex.
As a result, at the first stage, the air taken from the sewage treatment plant premises serves as a consumable for water purification. At the second stage, the cleaning agent is its own "dirt".
The final indicators for purification from petroleum products are 0.02 mg/l (the norm for drinking water is 0.05 mg/l), while the purified water visually looks cleaner than the original one. At the same time, there is a complete purification from suspended particles, hydrogen sulfide, iron, and manganese.
This scheme is very technologically advanced, has been working for more than 10 years and does not require loading and unloading consumables. Its main advantage is its ease of operation. An untrained person learns the procedure for cleaning oil-contaminated wastewater to the quality of spring water in two days.
The principle of operation of the coalescent filter is as follows. The water after purification on the flotation device enters the upper part of the filter. Next, it passes through a "perforated" filter layer of its own impurities, which passes only water molecules and does not pass through impurity molecules.
As the height of the filter layer increases (in about two to three days), the hydraulic resistance increases. The filter layer is drowned by a reverse flow of fresh water, the excess part is fed into the receiving tank.
This scheme makes it possible to introduce a closed water consumption cycle into various Russian Railways facilities, including wagon depots, and discharge only "purge water" into the sewer system. Closed-loop purging involves the introduction of a certain amount of fresh water so as not to accumulate mineral salts, the concentration of which increases due to evaporation processes.
Accordingly, when purging, it is necessary to discharge a certain amount of water, but it must be cleaned up to the standards. The coalescing filter that performs this task occupies an area of less than one square meter and easily fits into existing rooms.
Switching all Russian Railways enterprises to a closed cycle will save more than three billion rubles per year (fees for water consumption and sanitation). Today, this scheme has been implemented in the Novosokolniki, Volkhovstroy, Kandalaksha, Solvychegodsk, Petrozavodsk, Irkutsk, Predportovaya depot (St. Petersburg), Vyborg locomotive depot.
The method allows you to recoup the costs of a closed cycle in less than a year and completely eliminates environmental pollution from Russian Railways enterprises. At the same time, the purchase of additional chemicals, reagents, and consumables is not required, and operating costs will remain the same as before the introduction of the aftertreatment scheme.
29.06.2023, 22:14
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