Биохимическая очистка сточных вод. Обезвреживание рассолов.
Биохимическая очистка – один из основных методов глубокой очистки сточных вод, позволяющий удалить из них разнообразные органические и некоторые неорганические соединения при несложном аппаратурном оформлении процесса и сравнительно невысоких эксплуатационных затратах. При биохимической очистке растворенные органические вещества подвергаются с помощью микроорганизмов биологическому распаду в присутствии кислорода (аэробный процесс) или же в его отсутствии (анаэробный процесс).
При биохимической очистке вещества, содержащиеся в сточных водах, не утилизируют, а перерабатывают в избыточный ил, также требующий обезвреживания.
Активный ил представляет собой сложный комплекс микроорганизмов различных классов, простейших микроскопических червей, водорослей.
Существенное значение при использовании органических веществ микроорганизмами в качестве углерода имеет их строение. Насыщенные соединения – биологически стойкие и могут усваиваться только некоторыми видами микроорганизмов. Ненасыщенные органические соединения это хороший источник углерода для многих микроорганизмов.
Технологические схемы биохимической очистки сточных вод различны в зависимости от характеристик поступающих сточных вод и цели очистки.
Биологическое окисление проводят как в естественных условиях на полях фильтрации, орошения и в биологических трудах, так и в искусственно созданных условиях в аэротенках и на биофильтрах.
Аэротенки – емкостные проточные сооружения со свободно плавающим в объеме обрабатываемой воды активным илом, применяемые для аэробной биохимической очистки больших количеств сточных вод.
Секционирование аэротенки – железобетонные резервуары прямоугольные в плане и разделенные перегородками на 4-10 секции – коридоров. Длительность нахождения в аэротенке 6-12 часов. Из аэротенков смесь обработанной сточной воды и активного ила поступает во вторичный отстойник, где активный ил оседает на дно отстойника, а затем с помощью специальных устройств (илососов) отводится в резервуар. Так как в процессе биологического окисления происходит прирост биомассы активного ила, то её избыток выводят из системы и направляют на иловые площадки и сооружения по обработке осадка.
В биологических фильтрах активная биомасса закреплена на неподвижном материале, а сточная вода обтекает его тонким пленочным слоем. Их применяют при расходе сточных вод до 20 – 30 тыс. м3 /сут.
По типу загрузочного материала, на поверхности которого проходят биологическое окисление загрязнений, различают биофильтры с объемной и плоскостной загрузкой. Объемный загрузочный материал – гравий, щебень, керамзит и.т.д., плоскостной – из пластмасс, керамики, металла, тканей.
Не растворимые в воде загрязнения образуют на поверхности биофильтра (загрузочного материала) биологическую пленку, заселенную организмами. В процессе работы биофильтров пленка отмирает, опадает и воспроизводится вновь. Очищенную в биофильтре воду хлорируют и направляют во вторичный отстойник, где отмершая пленка задерживается. В фильтр вводят биогенные элементы в виде солей азота и фосфора и по мере образования биопленки постепенно добавляют сточные воды для увеличения концентрации загрязнений.
Для обеспечения жизнедеятельности микроорганизмов сточная вода, поступающая на фильтр, должна содержать не более 25 мг/л нефтепродуктов, не более 1 г/л растворенных солей.
В биофильтрах с плоскостной загрузкой в качестве насадки используют закрепленные блоки из листовой пластмассы с развитой удельной поверхностью (90-110м2/м3) и высокой долей свободного объема (93-96%). Блоки могут быть выполнены из поливинилхлорида, полистирола, полиэтилена, полипропилена, полиамида.
В анаэробных условиях биологически перерабатываются твердые, полужидкие вещества и осадки. Основное преимущество анаэробного сбраживания это минимальное образование биологически активных твердых веществ.
Метантенки – сооружения для анаэробного сбраживания, работающие по принципу реакторов с полным перемешиваниям. Типовые метантенки имеют полезный объем одного резервуара 1000 – 8000 м3 . условно этот объем можно разделить на 4 части, выполняющие разные функции: для образования плавающей корки, для иловой воды, для собственно сбраживания, для уплотнения и дополнительной стабилизации осадка при хранении (до 60 сут) .
Доочистку - сточных вод применяют для удаления содержащихся в биологически очищенных сточных вод частиц активного ила, биопленки, остаточных загрязнений органического происхождения, биогенных элементов (N и P), бактериальных загрязнений, затрудняющих повторное использование сточных вод в системах оборотного водообеспечения.
Characteristics of technological processes of oil refining plants as a source of water resources pollution. Protection of water pool from pollution. Normalization of water consumption at the enterprises
Oil refining and chemical industries are related to water capacious industries and they concede on water use only to power, metallurgy, the chemical industry and public services. The most part of water is used to cool and to condensate, and the products is also used as solvent or reagent as steam. Having passed this or that cycle, water is undergoes various changes, or it is irrevocably lost. Sewage contain both soluble, insoluble organic and inorganic substances, including toxic ones.
Oil and oil products are related to number of the most widespread and dangerous pollutant substances. In addition to hydrocarbons the sewage contains O-, S- and N-containing compounds.
The main principles of sewage treatment is necessity of the maximal reduction of sewage amount and decrease of impurities contents in them; a possibility to extract the valuable components from sewage and their subsequent reclamation; a reuse of sewage in technological processes and in the systems of circulating water supply.
Normalization of the harmful substances, disposal with sewage.
Before disposal in a pond sewage should be cleaned up to the highest degree because under effect of natural factors the secondary products of decomposition may be formed, negatively influencing quality of water.
To carry out process it is necessary, firstly, after sewage disposal, the presence in the pond of the soluble oxygen. Chemical or bacterial oxidation of the organic substances contained in the sewage, results in decrease in concentration of oxygen, dissolved in water. (1 liter of H2O contains 8-9 sm3 of O2. Organic substances interacting with dissolved O2 are oxidized to СО2 and Н2О. Therefore, the parameter was entered allowing estimating the total quantity of pollution in water according to absorption of О2. Such parameter is biochemical demand of oxygen (BOD). BOD is equaled to quantity of O2. Depending on time there are BOD5 (five-day), BOD20 (twenty-day), BOD (full), in this case the oxidation is finished.
According to « Rules of surface waters protection from pollution » the water quality of a reservoir after sewage disposal should meet to the following basic requirements: quantity of dissolved О2 should be not less than 4 mg / l; BOD (full) at 20 С is not higher than 3 mg/l. The contents of weighed substances in water after sewage discharge should not be increased more than in 0,25 and 0,75 mg /l for reservoirs I and II correspondently, a mineral sediment is no more than 100 mg /l; including chlorides is 350, sulfates is 500 mg / l; odour and smacks of water should absent; a surface of water should not contain the floating impurities, films, spots, oils, oil products, poisonous substances.
Reduction of water use.
OR and PC industries are related to water-capacious ones. The most part of water is consumed to cool and condensate the product stream. In a significant part of technological processes water is used as solvent or it is entered as a steam.
The main ways of improvement of water supply of the industrial enterprises are following:
-development of new technologies, which result in reduction of water use and which totally makes it possible to exclude water from technological process;
-sewage deactivation including extraction and reclamation of the valuable components; preparation of cleaned water to reuse.
-organization of the closed circulating water supply, including gathering and use of the cleaned sewage, floodwater and an atmospheric precipitation.
Essential decrease in water use is reached by replacement of water cooling by air. Air cooling makes it possible to reduce water discharge during working enterprises on 60-75 % and thus, volume of sewage is reduced on 25-45 %.
The organization of circulating water supply.
The basic way of discharge reduction of polluted and conditionally clean waters in reservoirs is a reuse, that is, the organization of circulating water supply.
During once-through run-of-river cooling, all water which is taken away from a reservoir, after use in technological process, is returned back into the reservoir excepting irrevocable losses in production and losses with a sludge at the purification structure (а), sewage after their cleaning (б). Not polluted heated sewage are supplied on cooling installations, and then they are returned to the circulating water-supply. The polluted sewage is entered at the purification structure.
At present 92-98 % of water are reused. In separate production this parameter has reached 100 %, that is, water is reusable used without disposal of polluted drains in reservoirs, and fresh water is added in connection with a natural loss (evaporations, chemical transformations, etc.)
Schemes of water supply of industrial enterprises:
а is an once-through; б is a circulating water; 1 is an enterprise; 2 are purification structures; 3 are cooling installations; I is a water from source; II are irrevocable loses of water; III is a water, removing with sludge; IV is a water, discharging in the reservoir; V is water loses for evaporation and ablation; VI is a circulating water after cooling installations; VII is a circulating water after purification structures.
a б
When using circulating water supply it is necessary to envisage the methods for removal of carbonate sedimentation, biological growth, corrosion of equipments and also the methods of preparation of make-up water.
Salts, collecting in the circulating water, form on the heat-exchange surface so-called carbonate sedimentations, more than on 50 % consisting of carbonate Са. The basic methods of struggle against them are treatment of cooling water by acid to decrease the total alkalinity; phosphatization by means of hexametaphosphate of Na, which stops processes of crystallization and sedimentation of carbonate Са on the walls of equipment; treatment of water by a magnetic field.
Methods of sewage treatment can be divided into 3 groups: mechanical, physical - chemical and biochemical. The general scheme of sewage treatment is shown on the drawing 1.
Mechanical and physical and chemical methods of sewage treatment.
Mechanical methods of sewage treatment are applied to extract insoluble mineral and organic impurities, the weighed particles in the size more than 5-10 mkm.
At first sewage are directed to the grating of purification structures to retain the large impurities, containing in them.
Sedimentation.
Rough mineral impurity and 90-95 % of floating oil products are retained in the sand traps, which are horizontal reservoirs, made from mass reinforced concrete. In the bottom of sand traps the clean-out pit is mounted, where the sediment is collected. To treat (clean) sewage containing more 100mg|l of no emulsifying hydrocarbons (oil, oil products), and also fine mineral impurities, it is possible to apply oil traps of the various designs. These are rectangular tanks in which separation of oil and water takes place due to a difference of their density.
Additional gravitational sewage treatment and averaging of their compound are carried out in the balancing reservoir or radial settling tanks, which are equipped by devices to collect oil products and to rake up sediment. The general mass of weighed particles (40-60 %) drops out in sediment for 1.5 hour. After cleaning in ponds of additional sedimentation the residual contents of oil products composites 70-100 mg / l, that is, it is decreased on 30-50 %.
-vertical settling tanks are applied at the discharge of sewage up to 10 000 m3/d.
- horizontal settling tanks are applied at the discharge of sewage up to 15 000 m3/d.
-radial settling tanks are applied at the discharge of sewage up to 20 000 m3/d.
Filtration. Emulsifying hydrocarbons and suspensions are removed by means of filtration on quartz sand, crush anthracite, quartz, a marble, a ceramic crumb, dolomite, polymer, etc.
Sandy filters are the steel vertical or horizontal tanks loaded by quartz sand by a layer 1м and operating under pressure 0,6МПа. Velocity of a filtration is 5-12 km/h. Efficiency of sewage treatment is 30-36 %. The residual contents of oil products is 45-55 mg /l. Low efficiency of sewage treatment is connected to plugging filtering checker by organic substances which can not be removed during washing.
Filters with floating media from polystyrene are single-layer and two-layer. Velocity of a filtration is 0.6-2 km/h. To regenerate granular filtering materials the intensive water-air washing is carried out.
To treat the sewage the filters «Polymer» with foam polyurethane media were developed. Residual contents of oil products is 10 mg /l, the velocity of filtration is 25-35 km/h.
Flotation.
Sewage treatment by flotation is to extract not dissolved impurities by means of finely dispersed air in sewage. Flotation installations are used to remove oil, oil products, and surface-active substances, solid particles, fiber materials from sewage.
Flotation is adhesion of particles of impurity and bubbles of air. Complex of bubbles-particles float on the surface. In this case particles are collected in the formed foam layer, which is removed from surface of water. There are some methods of flotation differing in the way of dispergation of air in the water.
Pressure head flotation is widely used. In this case the part of sewage is saturated by air under pressure. In order to intensify formation of the bubbles-particle complexes it is necessary to add in the water the different reagents increasing water-repellency treatment of particles surface. The degree of cleaning by flotation is 11 - 23 %.
The combination of pressure head flotation with coagulant treatment makes it possible to increase the sewage treatment degree from oil products on 90-95 % and mechanical suspensions is on 85-95 %.
When using pressure head flotation it is possible to use once-through or recirculation schemes. In case of once-through flotation the all sewage is directed to the carbonator, filled with a packer to intensify the process of dissolution air in water.
Flotation in a combination with coagulants and flocculants use (reagent flotation) is one of perspective method of sewage treatment and tertiary treatment. Thus, flotation makes it possible to treat sewage up to residual contents of pollution of 10-30 mg /l.
Biochemical sewage treatment.
Biochemical cleaning is one of the basic methods of the deep sewage treatment, allowing removing from them the various organic and some inorganic compounds. During biochemical cleaning the dissolved organic substances are subjected to biological disentigration by means of microorganism in the presence of oxygen (aerobic process) or in its absence (anaerobic process).
During biochemical cleaning the substances, contained in sewage, are not utilized, and they are processed into excess sludge which it is necessary to neutralize.
Activated sludge is a complicated complex of microorganisms of the various classes, the elementary microscopic worms, water plants.
When assimilating organic substances by microorganism the structure of compounds has essential value. Saturated compounds are biologically resistant and they may be assimilated only by some types of microorganisms. Nonsaturated organic compounds are good source of carbon for many microorganisms.
Depending on characteristics of sewage and the purpose of cleaning the technological scheme of biochemical sewage treatment are differentiated.
Biological oxidation is carried out both in natural conditions on fields of a filtration, irrigation and in biological ponds and in the artificial conditions in аerotanks and on the biofilters.
Aerotank is capacitive flow –type structure with freely floating activated sludge in the volume of treating water. For aerobic biochemical treatment, large amount of sewage is used.
Sectioning aerotanks are reinforced concrete reservoirs, divided into 4-10 section. From аerotanks the mixture of treated sewage is directed to the secondary settling tank, where activated sludge settles on the bottom of settling tank and then it is removed in the reservoir by means of disposal truck. Since during biological oxidation the growth of biomass takes place, its excess is removed from system and it is directed to the sludge site.
In biological filters, the activated biomass is fixed on a motionless material, and sewage flows around it’s by thin film layer. They are applied at the discharge of sewage to 20 - 30 000 m3/day.
According to type of the media material on which surface the biological oxidation of pollution proceeds, there are biofilters with volumetric and plane media. Volumetric media materials are gravel, crushed stone, expanded clay aggregate; plane are made from plastic, ceramics, metal, fabrics.
Not soluble pollution in water form the biological film, populated with organisms on a surface of the biofilter. During work of biofilters the film dies off, falls down and it is regenerated again. The cleaned water in the biofilter is chlorinated and directed to the secondary settling tank, where died film is delayed. In the filter the biogenic elements is added as salts of nitrogen and phosphorus. To increase concentration of pollution it is necessary to add the sewage.
In order to provide the life of microorganisms the sewage, supplying on the filter, should contain no more than 25 mg /l of oil products and no more than 1 g/l of the dissolved salts.
In biofilters with plane media the fixed blocks from sheet plastic with the advanced specific surface (90-110м2/m3) and a high share of free volume (93-96 %) are used as packing. Blocks can be performed from polyvinyl chloride, polystyrene, polyethylene, polypropylene and polyamide.
Methane tanks are constructions for anaerobic fermentation, working according to principle of reactors with full agitation. Typical methane tanks have useful volume of one tank 1000 - 8000 m3. This volume can be divided into 4 parts which have different functions: formation of floating crust, sludge water, fermentation, condensation and additional stabilization of sediment at storage (up to 60 days).
Tertiary treatment is applied to remove particles of activated sludge, biofilm, residual organic pollution, biogenic elements (N and P) and the bacterial pollution, contained in biologically cleaned sewage and complicating a reuse of sewage in systems of circulating water supply.
The methods of sewage treatment
1 is installation of mechanical cleaning;
2 is installation of biochemical or physical and chemical cleaning;
3 is installation of tertiary treatment;
4 is installation on disinfection sewage;
5 is structures for sediment treatment;
I is noncleaned sewage;
II is cleaned sewage;
III is sediment or excess biomass;
IV is treated sediment.