When concrete mixer trucks, pump trucks and mixers are washed, a large amount of sewage will be formed. According to relevant statistics, a concrete enterprise with an annual production capacity of 200,000 cubic meters can produce up to 30m3 of waste water every day.
At the same time, these wastewaters are not only highly alkaline, which will cause soil alkalinization and pollution of water resources if they are directly discharged, but also contain sand, stones and unhydrated cementitious materials. It will block various municipal facilities and pollute the river, which will seriously endanger the social environment.
In JGJ/T328-2014 “Green Production and Management Technology of Ready Mixed Concrete”, “wastewater treatment system” and “wastewater utilization” are regarded as the scoring items of green production star rating.
In view of these situations, it is necessary to treat and recycle these waste water and replace it with water for concrete production. On the basis of ensuring the quality of concrete, it can also purify the environmental sanitation of the mixing plant, protect the ecological environment, save water resources, and bring certain economic benefits. Realize the “zero discharge” of waste water and waste residue.
Basic properties of wastewater discharged from concrete batching plants
After the concrete is produced in the mixing station, the tap water is generally used to wash the mixing equipment and the mixer truck. The generated wastewater contains not only incompletely hydrated cement, unhydrated admixtures, clay, fine sand and other granular solids , and also contains many hydration ions, such as Ca2+, OH, etc.
To a certain extent, the solids content is determined by the amount of flushing water and the treatment facility, the ionic concentration is affected by the concrete design mix, and the wastewater properties are affected by residual admixtures.
When testing wastewater, the test index is mainly solid content. Relevant studies have shown that the solid content of wastewater has no relationship with its pH. When the storage time is prolonged, various wastewater particles will continue to hydrate and change the ion concentration.
When wastewater is used, wastewater ions will affect the hydration of cementitious materials, resulting in shortened setting time of concrete.
At this stage, in most of the wastewater treatment systems of batching plants, sand and stones are separated by filtration through sand and gravel separation equipment, and the wastewater is then separated and precipitated. When precipitating wastewater, it often passes through multiple sedimentation tanks. After the solid particles in the wastewater are precipitated many times, an ideal precipitation effect can be obtained.
However, it covers a large area, and the waste water and waste residue are not used, which does not meet the requirements of green environmental protection.
Our station is filtered through sand and gravel separation equipment to separate sand and stone for recycling. The waste water is directly discharged into the stirring tank (two hexagonal tanks with a diameter of 3.5m and a depth of 4.0m connected with each other and installed with stirring blades), and stored for future use.
Influence test of clean water and waste water on the performance and strength of concrete
(1) Cement: Tower brand P.O42.5R cement, the 28-day compressive strength is 50.2MPa.
(2) Machine-made sand: medium sand, fineness modulus 2.7, powder content 4.6%.
(3) Crushed stone: 5mm~31.5mm particle size, continuous gradation.
(4) Mineral powder: Fujian Sanming mineral powder, S95.
(5) Admixture: finely ground limestone powder, 0.045mm sieve residue <15%. (6) Clear water: tap water. (7) Wastewater: Wastewater with a solid content of 10%. (8) Water reducing agent: Huaxinda high-efficiency water reducing agent, the water reducing rate is 26%. ><15%.
(6) Clear water: tap water.
(7) Wastewater: Wastewater with a solid content of 10%.
(8) Water reducing agent: Huaxinda high-efficiency water reducing agent, the water reducing rate is 26%.
As can be seen from Table 1, when wastewater with a solid content of 10% is used as mixing water, although the amount of admixtures has increased (the solid content of wastewater contains incompletely hydrated cement, unhydrated admixtures, clay, etc.). , fine sand and other granular solids, which will adsorb admixtures), but can improve the workability of concrete, and have little effect on slump and slump loss, setting time and strength, and can be used completely.
It can be seen from Table 3 that the carbonization coefficients of concrete members stirred with clean water and wastewater are almost the same, which is regarded as having no effect.
My specific practices in use
(1) After the sand and stone are separated, the waste water is directly discharged into the stirring hexagonal tank for stirring;
(2) Keep the water level in the stirring hexagonal tank within a certain range. When the production volume is large and the waste water is insufficient, supplement it with clean water, that is, control the relatively stable solid content;
(3) An automatic stirring device is installed in the stirring hexagonal tank (stir for 5 minutes, stop for 90 minutes, and stir according to the set program if it is not in production);
(4) Cancel the original clean water intermediate silo, directly extract the waste water to the scale for measurement, to prevent sedimentation at the bottom of the intermediate water silo;
(5) The solid content is measured every 2 hours before the opening of the market every day and during the production period, so as to adjust the concrete mix ratio in time and ensure the quality of the delivered concrete.
(1) When all the wastewater with a solid content of 10% is used as the mixing water, the dosage of the admixture is increased by about 0.2%~0.3%.
(2) The workability of concrete becomes better, and the sand rate can be appropriately reduced by about 1%.
(3) It has little effect on slump and slump loss, setting time and strength.
(4) Further tests are required for the carbonization performance and durability of concrete components.
(5) Since the mixing tank realizes continuous automatic control mixing, the placement time of the waste water has little effect on the performance of the concrete.
The concrete batching plant can recover the waste water for recycling in the concrete preparation through the waste water recycling system. By using a sand and stone separation system with a simple overall structure and easy installation and use, a stirring hexagonal tank with an automatic stirring device is installed.
The solid content of wastewater is relatively stably controlled, and the wastewater of the mixing plant is recycled and reused to achieve zero discharge of wastewater and waste residue, which not only avoids waste of resources, but also reduces the actual production cost of the concrete mixing plant.
It also reduces the shortage of many sedimentation tanks and covers a large area, and meets the requirements of a green and environmentally friendly mixing station.