The acidic wastewater from aluminum plate production mainly comes from the acid etching process, neutralization, oxidation process, spray pre-treatment oil removal, acid washing and other processes in the oxidation workshop, which are rich in various harmful substances or heavy metal salts. The mass fraction of acid varies greatly, with low values less than 1% and high values greater than 10%. The alkaline wastewater mainly comes from processes such as alkali corrosion in the oxidation workshop and alkali washing for spray pre-treatment. The mass fraction of alkali is sometimes higher than 5% and sometimes lower than 1%. Spraying and dyeing also generate wastewater. In addition to being rich in acid and alkali, wastewater often contains oil, paint, fluoride salts, and other inorganic and organic substances.
Acid alkali wastewater has strong corrosiveness and needs to be properly managed before being discharged. The general guidelines for managing acid-base wastewater are:
① High concentration acid-base wastewater should be prioritized for recycling and reuse. Depending on the water quality, quantity, and different technical requirements, it should be reused as much as possible. If reuse is difficult, or if the concentration is low and the water volume is large, concentrated methods can be used to recycle the acid-base.
② Low concentration acid-base wastewater such as cleaning water for acid pickling tanks and rinsing water for alkaline washing tanks should be neutralized.
When it comes to neutralization disposal, the primary consideration should be the principle of treating waste with waste. Neutralize acidic and alkaline wastewater with each other or use waste alkali (slag) to neutralize acidic wastewater, and use waste acid to neutralize alkaline wastewater. When these conditions are not met, neutralizing agents can be used for disposal.
According to the national standard GB8978-1996 "Emission Limits for Water Pollutants", the emission requirements are: COD1 level ≤ 60mg/L, level 2 ≤ 120mg/L, suspended solids ≤ 100mg/L, fluoride ion F - ≤ 10mg/L, pH value 6-9.
Modern wastewater treatment methods are primarily divided into three categories: physical treatment, chemical treatment, and biological treatment.
1) The physical disposal method is a wastewater treatment method that separates and recovers insoluble suspended pollutants (including oil films and oil droplets) from wastewater through physical effects. Generally, sedimentation, filtration, centrifugation, air flotation, transpiration crystallization, reverse osmosis and other methods are used. Separate pollutants such as suspended solids, colloids, and oils from wastewater to initiate purification.
2) Chemical disposal is a wastewater treatment method that uses chemical reactions and mass transfer effects to separate and remove dissolved or colloidal pollutants from wastewater, or convert them into harmless substances. The general selection methods include neutralization, coagulation, oxidation recovery, extraction, stripping, blowing off, adsorption, ion exchange, and electro infiltration.
3) Biological disposal is a wastewater treatment method that utilizes the metabolic effects of microorganisms to convert pollutants such as wastewater solutions, colloids, and fine suspended organic and toxic substances into stable and harmless substances. Biological disposal methods are divided into two types: aerobic disposal and anaerobic disposal. The commonly used aerobic disposal methods currently include activated sludge process, biofilter, and oxidation pond. Anaerobic treatment method, also known as bioremediation treatment method, is primarily used for treating high concentration organic wastewater and sludge, using disposal equipment such as digestion tanks.
The intention of disposing of sludge is to:
① Reduce the moisture content of sludge to create conditions for the disposal, utilization, and transportation of sludge.
② Eliminate harmful substances that pollute the environment.
③ Retrieve motivation and capital to turn harm into profit. The methods for sludge disposal include sludge concentration, sludge digestion, sludge dewatering, and sludge drying. The intention of sludge concentration is to initiate dewatering and reduce the volume of sludge, providing conditions for subsequent disposal. The intention of sludge dewatering is to further dewater and reduce the moisture content in the sludge to below 80%. There are two methods for this: mechanical dehydration and natural dehydration. Mechanical dehydration method is divided into vacuum suction filtration method, pressure filtration method, and centrifugation method. Its advantages are high dehydration rate, small footprint, but relatively expensive investment. The construction and operation costs of natural drying method are very low, but the dehydration power is low, the land area is large, and the hygiene environment is poor. The purpose of sludge drying is to heat the dehydrated sludge, further reducing its moisture content and volume. The commonly used rotary drum type drying machine for dry disposal method has the advantages of stable function and reliable operation, but it occupies a large area.
