Water-Quality Characteristics and Treatment Solutions for Petrochemical Wastewater
The wastewater discharged from the production of synthetic rubber, synthetic plastics, fibres, detergents and other products, as well as chemical raw materials such as benzene, naphthalene, methanol, glycerin and acetaldehyde, contains raw materials, products and by-products; it has a high organic-matter content and emits harmful odours, and requires sedimentation, biochemical treatment or ozonation treatment, activated-carbon adsorption treatment, etc. After tertiary treatment, the wastewater can meet higher environmental-hygiene standards or be reused in production.
Research and application of polymer flocculants:
Inorganic polymer flocculants, such as polyaluminium and polyferric, have been widely used in China with good results, gradually replacing traditional inorganic-salt flocculants.
Compared with inorganic flocculants, organic polymer flocculants have the following advantages: low dosage (1/10-1/40 of inorganic flocculants), wide application range (pH 4-9), good purification effect, low sludge generation and low water content, and no increase in the salt content of water or the metal-ion content of sludge, which favours water reclamation. Many US refineries and petrochemical plants have fully replaced inorganic with organic flocculants.
Organic polymer flocculants are divided into three types — anionic (polyacrylamide, sodium polyacrylate), cationic (polyamine-type, quaternary-ammonium-type, copolymer-type) and non-ionic (polyacrylamide, polyoxyethylene, water-soluble urea resin) — among which the cationic type is more suitable for oily-wastewater treatment.
In the research and production of organic polymer flocculants in China, for a previous period they were limited to anionic and non-ionic types, and only a few such as polyacrylamide and carboxymethyl cellulose were sold commercially. In recent years, some Chinese universities and research institutes have begun to develop cationic polymer organic flocculants, several of which, such as cationic acrylamide copolymers, have entered organised production. However, these are mainly suitable for the flocculation of wastewater with high suspended-solids content and for sludge dewatering, and are not very suitable for treating the oily water of refineries and petrochemical plants.
China's refineries and petrochemical plants are essentially still limited to using inorganic flocculants (including inorganic-polymer types); some refineries have tried combining inorganic flocculants with anionic organic polymer coagulants, but because too few organic flocculant varieties are available and the application technology is not well mastered, no stable, definitive conclusions have yet been reached.
Because China's refining and petrochemical enterprises basically use inorganic flocculants for wastewater treatment, the problems of large sludge generation and difficult treatment are inevitable; the research and development of organic polymer flocculants has become an urgent priority. At present, the first step is to rapidly organise pilot-scale production of the successfully developed organic polymer flocculants for oily-wastewater treatment and gain experience through field use. For different treatment targets, the suitable flocculant type and variety also differ; using flocculants correctly is, in a sense, an 'art', and field testing is often decisive, so we should strengthen the R&D of organic flocculants and achieve basic coverage of types and main varieties in the near term.
Tests concluded: in the coalescing-filtration process, oil droplets >15 um are basically removed and those <10 um are also removed by 60%; after two-stage coalescing filtration, the oil content of the wastewater drops from 25-142 to 6-32 mg/L, and the oil-removal effect is better than flotation (effluent oil content 1-51 mg/L), meeting the influent-quality requirements for biological treatment. In addition, compared with flotation, the sludge is reduced by 70%, electricity saved by 30% and water-treatment cost lowered by 31%.
Coalescing filtration uses a material with a rough surface, strong oil adhesion, moderate particle size and good strength as the coalescent, packed in the bed, to perform coalescing filtration on oily wastewater. The process can be divided into three stages: 1. Oil-film initial stage — fine droplets in the oily wastewater passing through the bed are captured by the coalescent and spread on its surface to form an oil film; 2. Oil-film thickening stage — as more oil droplets are captured, the oil film thickens and is retained in the bed voids; 3. Film-release stage — the coalesced and agglomerated oil in the bed is dragged forward by the water flowing through it. Coalescing oil removal mainly uses stages 1 and 2. After entering stage 3, the oil content in the effluent begins to rise. At this point operation should be stopped for backwashing, so that the adhered oil and suspended matter fall off the coalescent surface, form larger particles, and are separated by gravity settling.
This process features a simple flow, easy operation and management, a compact unit and small footprint, creating favourable conditions for automatic control.
When the coalescing-filtration method treats low-emulsification oily wastewater, no flocculant needs to be dosed; when the wastewater has many surface-active sites, a small amount of flocculant should be dosed for destabilisation and coalescence.
The highly emulsified wastewater (e.g. diesel alkaline-refining wash water, heavy-oil and dirty-oil-tank drains, tank-washing-station wash water) generated by refineries and petrochemical plants during production, when mixed with oily wastewater, turns the originally lightly emulsified wastewater into severely emulsified wastewater, disrupting the normal operation of oil separation and flotation. The commonly used methods for treating emulsified-oil wastewater — heating, acidification and dosing demulsifiers — each have problems of high energy consumption and, for acidification (pH<3), considerable chemical consumption, and the demulsification effect is often unsatisfactory. Tests have shown that the alternating asymmetric pulse electrocoagulation method achieves good results in treating emulsified-oil wastewater.
Microwave-radiation method for treating emulsified-oil wastewater
When microwave radiation treats emulsified-oil wastewater, under the microwave irradiation the ion movement in the emulsion intensifies and compresses the double electric layer, thereby lowering the Zeta potential and achieving demulsification.
Treatment of high-concentration and refractory (hard-to-biodegrade) wastewater
For the high-concentration wastewater discharged by petrochemical enterprises, those with good degradability are suitable for anaerobic biological treatment, while those that inhibit the biological-degradation process or are non-biodegradable are treated by chemical or physical methods.
The anaerobic biological method features low energy consumption and recoverable biogas as an energy source, low inorganic-nutrient demand, low treatment cost and little excess sludge. The high-efficiency bioreactors developed from the late 1970s to the 1980s have a high biomass in the bed and are suitable for treating high-concentration wastewater. In addition, the hydrolysis-fermentation stage of the anaerobic biological process is highly adaptable; after appropriate cultivation and acclimatisation it also achieves a fairly good degradation effect on refractory organic matter, and is increasingly valued in petrochemical-wastewater treatment.
(2) Chemical and physical methods of treatment
High-concentration wastewater that inhibits or is difficult to biodegrade is the main difficulty in petrochemical-wastewater treatment. Whether this portion of wastewater can be properly treated is the key to whether petrochemical-enterprise discharge meets the discharge standards. Both China and other countries attach great importance to this, and have taken measures for specific situations and developed some technologies, achieving certain results. However, there is still a considerable distance from a fundamental solution, and further experimental research and development of new technologies is needed.
