Water treatment in a filtering and regenerating bioplato
The comparative analysis of efficiency of deep additional water treatment in a hydroponic type filtration-regeneration bioplateo and bioplato of other designs is given.
It is shown that the application of middle drainage for permanent flushing with circulating-washing water of the plants' root system filter-fill and bioplato drainage and hydro-automatic treatment of circulating-washing water allows to provide self-restoring operation mode of the treatment facilities complex.
Filtration and regeneration bioplates provide an opportunity to achieve higher quality and stability of water treatment regardless of their capacity and climatic conditions of use. Key words: water treatment, biophytotechnology, higher aquatic plants, bio-platelets, filtration, self-cleaning filters.
Filtration bioplates as structures for treatment and purification of domestic, industrial waste water, surface runoff in recent years are becoming widespread. Their advantage is that they almost do not require energy and chemical reagents, significant operational maintenance and provide high quality water purification from a wide range of organic and mineral pollutants.
Complex interconnected aerobic and anaerobic processes take place in the bioplato system, accompanied by filtration, sorption, absorption and transformation by plants and microorganisms of various compounds and elements. The main mechanism of water purification on bioplates consists in the activity of heterotrophic and autotrophic organisms. Heterotrophs, which are represented primarily by bacteria, protozoa, absorb inorganic and organic substances from the aquatic environment and decompose them into simple compounds (carbon dioxide, water, nutrients, etc.). Autotrophs, represented above by aquatic plants and algae (VPP), absorb a number of inorganic elements, nutrients, carbon dioxide from water and use them in the construction of their own bodies, creating organic matter from them by photosynthesis.
Higher aquatic vegetation-macrophytes include broadleaf cattail, lake reed, susak, arrowroot, elodea, water lily, marsh calamus, rootless wolfia, pondweed and others, which may float on the surface and in the water column, or may be submerged in the soil of the pond.
A positive factor that significantly affects the purification is the formation of biofiltering biofilm on the surface of the filter bed and the root system of plants, where various microorganisms develop in the form of immobilized bacterial medium, due to whose activity organic substances and various toxic compounds are effectively decomposed and removed [1, 2; 3].
Such filtering biophytotechnologies allow the extraction of biogenic and inorganic elements (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur), heavy metals (cadmium, copper, lead, zinc), salt anions (chlorides, sulfates, nitrates), various organic substances (phenols, oil products, surfactants, products of vital functions of living organisms, plants, etc. The most frequently used bioplastics are bioplates.
Open water mirror bioswales are most commonly used in countries with warm and temperate climates. Depending on the region and plant species, the degree of purification of water from contaminants can be significant. In particular, according to researches in Great Britain average percentage reduction of pollutants concentration in domestic sewage is 48% for BOD, 83% for suspended solids, 51% for total nitrogen, 13% for phosphorus, 99% for pathogenic microorganisms. In the USA the degree of purification of domestic wastewater using water hyacinth by BOD5 reaches 97-98%. In China the treatment efficiency of silver, suspended solids, phosphorus and nitrogen compounds was 100%, 91%, 54% and 93%, respectively, with BOD and COD decreasing by 98% and 91%. The concentrations of chlorides and sulphates were slightly lower (up to 60%), as well as those of hardness salts and heavy metals (up to 37%). Bioplato is effectively used for treatment of domestic wastewater and surface runoff in the Netherlands, Japan, Norway, Australia and other countries [4; 5, 6; 7, 8].
However, in regions with warm climates, pests (including the larvae of the malarial mosquito) can breed in the open water of bioswales. If petroleum products enter, the open water mirror is blocked by a film, hindering the biological cleaning and aeration processes of the water body. In such bioswales no effective removal of heavy metals, arsenic, dioxins, drug residues, pesticides, surfactants is ensured, as they only partially accumulate in the biomass of water bodies. The presence of an open water mirror on the bioswales also causes harmful aerosols and unpleasant odours. If pathogens are present in open bioswales, they can be spread by waterfowl, insects and aquatic animals.
Open bioswales do not provide regulation of mass exchange, gas saturation and aeration of water, do not remove mineralized sludge and sediment. This leads to colmatation of drainage systems, filter backfill, silt accumulation in the bottom part of structures, development of anaerobic processes, becomes the cause of secondary water pollution and significantly reduces the effectiveness of pollution treatment. In addition, in regions with moderate climate during autumn-winter period due to decrease in temperature efficiency of open bioplants decreases on average by 30-40%.
At the Institute of Hydrobiology of the National Academy of Sciences of Ukraine, open bioplats of various designs for water treatment have been studied, which showed high efficiency. In particular, canals that transport water from the Dnieper River for water supply of such regions as Crimea, Donbass have been used as water treatment facilities . Extensive research and implementation of such facilities is being carried out at the Institute of Environmental Problems (Kharkiv) . In the State Ecological Academy of Postgraduate Education and Management (Kiev), there have been developed biofloors with floating backfill in the form of special rafts with planted in them HPF, in particular, broad-leaved cattail and swamp aire, which allow quite effectively purify water from oil products, grease, UAR, biogenic compounds of nitrogen and phosphorus, suspended matter . In these constructions combined horizontal-vertical movement of water is applied, it allows controlling filtration rate, increasing the duration of contact of pollutants with the root system
BBP improve sorption of various pollutants.
The Potential-4 Research and Engineering Centre (Kiev) has proposed the closure of hydroponic-type filtration bioplants (HFBP) . In them, the water level is below the upper level of the backfill, in which the HPBs are planted, and their root system is constantly washed by water, moving vertically from top to bottom or from bottom to top.
A WWTP combines the basic elements of treatment using immobilised microflora on inert bed and higher aquatic plants with diversion of treated return water into a water body directly or through a groundwater stream. A peculiarity of ZBGT is regulation of water quality by means of artificially created hydrobiocenosis, characteristics of which components are formed under the direct impact of WRD without open water surface. One of the variants of WBCP of this type is insulated biofloors, in the zone of water mirror or under/over the backfill of which thermal insulating fibrous materials are placed. This makes it possible to recover wastewater heat and to use WWTPs in regions with a temperate climate and in winter.
At the same time, in such WBCPs there is a gradual colmatation of the root system of WRD, pore space of the filter bed and drainage by biofilm and mineralized sediment, the accumulation of silt in the bottom of the structures, reduction of oxygen and nutrients to the root system of WRD, which can lead to reduced efficiency of structures, peptization of sediment and secondary water pollution. As sludge removal is not provided in the TSF, anaerobic biological processes begin to take place in the backfill as it compacts, resulting in reduced sorption and detoxification of toxic impurities. To restore the operation of bio-floats, their periodic shutdown for repair and restoration work associated with regeneration washing of filter backfill, the root system of WRD and drainage is necessary.
The purpose of this work is to develop and investigate hydroponic-type filtration and regeneration bioplates (FRBGT), in which continuous washing and regeneration of the filter bed, the root system of HPP and drainage are implemented, providing deep removal of various pollutants from water of multicomponent composition, improving mass exchange and aeration of HPP root system, stable and continuous operation of the complex of structures and increasing the reliability of their functioning in various climatic conditions.
Filtration and regeneration hydroponic biofilters (FRBGT-1), in which the water level is below the top level of the filtration bed, were investigated at the municipal wastewater treatment facilities in Quartz and Kamianka-Bugskaya. The average composition of raw sewage before biofiltering is given in table
Average composition of initial wastewater before biophytotreatment
The bio-float (FRBGT-1) consisted of a rectangular reinforced concrete tank (1) 13 m wide, 50 m long and 2.6 m deep, which contained two layers of filter backfill (2, 15). The upper layer of filter bed (2) consisted of washed granite-basalt rubble of 35-50 mm fraction, the lower layer (15) with washed granite-basalt rubble of 10-25 mm fraction. The thickness of all layers of filtering backfill was 2100 mm, in which the upper filtering granite-basalt layer was 1400 mm.
In the thickness of the filter bed (2 and 15) an upper (6), middle (5) and lower (4) drainage was installed, which were placed evenly over the whole area of the tank. The upper drain (6) of the source water distribution was placed in the root system of higher aquatic plants-macrophytes and moisture-loving trees (TLD) (8) and connected to the treatment water supply pipeline (1) as well as to the circulation-wash water and clarified wash water collector (pipeline) of the self-cleaning Styrofoam filter. The bottom drain (4) for collecting and draining the clarified water was placed at the bottom of the tank at the bottom of the filter bed. The middle drain (5) for collection and discharge of circulation-wash water was placed in the filter bed between the upper and lower drains.
Energy willow (40- 45%), reed (30-35%), broad-leaved cattail (15-10%), miscanthus (10- 5%), and water grass (5%), with root systems in the upper part of the filter bed (2) were used as WWTPs.
The self-cleaning polystyrene foam filter (11) for circulation-washing water filtration was designed as a cylindrical steel vessel 2800 m in diameter, 5800 mm in height, which housed a 1300 mm thick polystyrene foam granule filter backfill. The filter is equipped with a device (12) for biological washing of the filter bed. The filter wash water was stored and settled in two clarifiers (13) with a diameter of 2000mm and a total volume of 13 m3
Figure. Schematic diagram of FRBGT-1:
1 - bioplato body, 2 - top layer of filtering backfill, 3 - collector of water supply for treatment, 4 - bottom drainage of treated water outlet,
5 - middle drainage of circulation and leachate collection and discharge,
6 - upper drainage of water distribution in the inter-root VPR system,
7 - collector of withdrawn treated water (filtrate), 8 - higher aquatic plants and/or damp-loving trees (TLD), 9 - collector of circulating and leachate water, 10 - pump, 11 - self-cleaning Styrofoam filter, 12 - device for biological filter washing, 13 - clarified filter wash water intake, 14 - clarified wash water pump, 15 - bottom layer of filtering backfill
FWBGT-1 operates as follows. Water for purification is supplied through the collector (3) into the upper part of the bioplate body (1), distributed over its area and filtered from top to bottom through the upper layer of the filter bed (2). As in FRBGT-1 the water level is below the top level of the backfill, in which the HPFs are planted, their root system is constantly washed by water, moving vertically from top to bottom.
In the upper layer of the backfill (2), where the root system of the EPR is located (8), the suction and transport of water and mineral substances by the root system of higher aquatic plants, synthesis of biologically active substances, accumulation of food products withdrawn from water in the green biomass, reproduction of EPR and interaction of dissolved impurities removed from water with various aquatic organisms (bacteria, actinomycetes, fungi, protozoa, unicellular algae, etc., which are present in the water. The water is then filtered through the bottom layer. The water is then filtered through the bottom layer of the bedding (15). The purified water (filtrate) is collected by the lower drainage system (4), drained by the collector (7) into the contact tank and then discharged into a water body or sent to the consumer after additional treatment and disinfection.
As during water filtration the film of activated sludge, suspended mineral and organic impurities accumulate in the filter bed, such conditions are created, that part of the filtered water is constantly drained from the lower zone of the WRF root system by the middle drain (5) into the accumulator (9) and then periodically or continuously by the pump (10) is fed to the self-wash polystyrene foam filter (11) for desliming from suspended solids. As a result, intensive mass exchange occurs in the root-filter backfill and flushing of inter-root space of WRF (8).
The purified circulating and flushing water is returned to the inlet of the bioplate via the collector (3). The filter (11) is periodically flushed with a hydro automatic flushing device (12). The rinse water is discharged into the filter wash water clarifier (13), from where it is pumped to the collector (3) after sedimentation by the pump (14). Sludge and mineralised sludge from the clarifier (13) is periodically removed to the sludge and compost pads.
The duration of the studies was 3 phases of 12-30 days each during 18 months of production operation of the bio-platform. The following main options of bioplate operation were studied: 1 - without using middle drainage in the bioplate and a styrofoam filter (the bioplate - design analogue of ZBGT 'Potential-4' was studied); 2 - with cyclic (periodic) washing of the bioplate's filter backfill using a self-flushing styrofoam filter; 3 - with continuous washing of the bioplate's filter backfill using a self-flushing styrofoam filter.
The results of bioplateo operation in different variants showed the following. When the bioplate works according to the first variant when the periodic washing of the filtering backfill by stopping the work of the facilities within 40-90 minutes and drainage of washing water 6 times within the period of the study and 3 times within the period of the bioplate setting allows to get rather stable results of water purification.
However, the significant disadvantages of such biofilter water purification is the need for periodic stops of bioplato operation for regeneration of filter mat and continuous monitoring of the structures by qualified service personnel. After start-up, such plants are in operation for a long period of time according to the design parameters, including the quality of treated water. With high content of suspended solids in the source water there is a rapid and uncontrolled colmatation of the root system of WRF, drainage and filtering backfill bioplato, which leads to deterioration of water treatment quality due to development of anaerobic processes, deterioration of mass exchange of root system with "raw" water, significant complication of control over the complex of structures at night and especially during periods of long rainfall, early snowfall, flooding.
The operation of the FRBGT-1 under the second variant was carried out without stopping the water supply for treatment and with periodic washing of the filter bed of the bioplate. This variant allows obtaining more stable results of water treatment compared to the first variant. The efficiency of water treatment by this method on some parameters compared to the previous option was on average 20-30% higher.
The disadvantages of the second option of water treatment are the necessity of manual setting of modes and change of periodicity of washing cycles of filter backfill bio plateau and permanent monitoring of the complex of facilities at fluctuations of water quality indicators at the inlet before the bio plateau.
Filtration and regeneration bio-plateaus after start-up without subjective intervention come into operation according to the design parameters, but there is a need for additional and continuous monitoring of the process of filter backfill washing to ensure stable water purification. When water with a higher content of suspended solids is supplied to the inlet of the bioplate, short-term colmatation of the drain, the root system of the WWTP and the filter bed of the bioplate may occur with deterioration of the control parameters of pollutants in the filtrate.
The operation of FRBGT-1 according to the third variant was carried out by constant washing of the filtering backfill of the bioplate without the influence of the subjective factor and stopping of water supply for treatment and stopping of its work. This option showed stable and better results. Compared to the previous versions, it improved the degree of water purification by 50-70%, especially when feeding water with a higher content of suspended solids and biogenic compounds of nitrogen and phosphorus through improved mass transfer processes in the root system of filter bed and permanent washing out of mineralized film of activated sludge. It provides deep water treatment in automatic self-regulating mode and allows you to clean water without having to technological stoppages for washing the biofilter bed and to abandon the service personnel during the whole period of CWBGT-1 complex of structures.
The degree of wastewater treatment during operation of the biological plateau in different variants is shown in Table 2. As can be seen from the results of research, the highest degree of purification was observed in the third option of wastewater treatment using the average drainage and continuous automatic washing of the filtering backfill bioplate with circulating and flushing water and its cleaning on a self-cleaning polystyrene foam filter.
Thus, the results of studies have shown that the use of filtration-regenerative hydroponic type bioplato (FRBGT-1) compared with the known technological schemes of biofilter water treatment and designs of bioplato allows, due to continuous automatic washing of filter backfill, the root system and drains of bioplato and biological treatment of circulating and leaching water to provide self-recovery mode of the complex of treatment facilities. FRBGT-1 make it possible to achieve higher quality and stability of reagent-free water treatment regardless of the size and capacity of the bio-plateau, cyclicity of water supply, climatic conditions, to abandon the service personnel and continuous monitoring of the biofilter water treatment plant complex operation.
Degree of wastewater treatment for different FRBGT applications
Note: The raw water parameters are given in Table 1