When dealing with sludge, everyone must have encountered this headache: how much PAM (polyacrylamide) should be added to make the sludge dehydrate properly? If added too little, the sludge becomes sticky like "brown sugar" and the dewatering effect is poor; Too much, not only will the cost continue to rise, but it may also affect subsequent processing. Today we talked about how to determine the amount of PAM to be added during sludge dewatering by breaking and crushing it apart!
First, understand what PAM is as a 'artifact'
PAM, also known as polyacrylamide, is like a "binder" and "commander" in sludge dewatering. It has three types: cationic, anionic, and non-ionic, with different "personalities" corresponding to different types of sludge. Cationic PAM is good at treating negatively charged organic sludge, such as sludge from domestic sewage treatment plants, as well as sludge from food and paper mills; Anionic PAM has a special preference for inorganic sludge with positive charges, such as sludge produced in metallurgy and mineral processing; Non ionic PAM is suitable for a wide range of sludge types, and can be tried when the sludge properties are not clear or when the acidity or alkalinity of the sludge is moderate.
PAM can work by two abilities: one is called adsorption bridging, whose long molecular chains can "string" small sludge particles together to form larger flocs; Another method is called electrostatic neutralization, which neutralizes the surface charge of sludge particles so that they no longer repel each other but tightly embrace each other, making it easier for water to be separated.
The 'behind the scenes' mastermind affecting PAM dosage
1. The "temperament" of sludge
-Sludge type: There is a huge difference between organic sludge and inorganic sludge. Organic sludge contains a lot of organic matter, with small and negatively charged particles. Generally, cationic PAM is used, and the dosage is relatively high, possibly 5-20 milligrams per liter of sludge; Inorganic sludge particles are relatively large, and anionic PAM can be used, with a dosage of 1-5 milligrams per liter. For example, the remaining sludge from municipal sewage treatment plants is mostly organic sludge. When adding cationic PAM, the dosage is often between 8-15 milligrams per liter.
-Sludge concentration: The thicker the sludge, that is, the higher the concentration, the more PAM is required. For example, compared to sludge with a concentration of 5% and a concentration of 1%, in order for sludge with a concentration of 5% to achieve the same dewatering effect, the amount of PAM added may need to be doubled or even more. Due to the high concentration, there are more sludge particles inside, requiring more PAM molecules to "connect" and "neutralize".
-The pH value of sludge: Different types of PAM have different preferences for pH value. Cationic PAM performs well in neutral to acidic environments, while anionic PAM is more effective in neutral to alkaline environments. If the pH value of the sludge is not suitable, it may be necessary to adjust the pH value first and then add PAM, otherwise the effect will not be good no matter how much is added. For example, adding ordinary cationic PAM directly to the alkaline sludge of a printing and dyeing factory will definitely not be effective. You need to first lower the pH value or choose alkaline resistant cationic PAM products.
2. Significant differences in processing techniques
-Dehydration equipment: The amount of PAM added varies depending on the dehydration equipment used. The centrifugal dewatering machine has a fast speed and high shear force. In order to ensure that the flocs are not dispersed, the amount of PAM added should be slightly higher, usually between 8-15 milligrams per liter; The shear force of the belt filter press is relatively small, and the dosage may be between 5-10 milligrams per liter; The plate and frame filter press does not require high floc strength, and the dosage is usually 3-8 milligrams per liter.
-Mixing method and speed: When PAM is mixed with sludge, if the mixing is not uniform, there will be more PAM in some places and less PAM in others, which will affect the overall effect. And the mixing speed is also crucial, too fast will break the newly formed flocs, and too slow will result in insufficient mixing. Usually, at the beginning, stir quickly for 1-2 minutes to allow the PAM to disperse quickly, and then stir slowly for 3-5 minutes to allow the flocs to slowly grow.
Practical methods for determining the dosage of PAM
1. Laboratory trial "exploration"
-Preparation: Take multiple clean beakers and fill them with equal amounts of sludge samples. At the same time, prepare PAM into a solution with a concentration of 0.1% -0.5%. The concentration should not be too high, otherwise PAM will easily clump and dissolve poorly.
-Addition test: Add different amounts of PAM solution to each beaker, starting from 1 mg/L and increasing by 1 mg/L each time, until reaching 10 mg/L or more. After adding, use a mixer to stir at the speed and time mentioned earlier.
-Observation record: Don't worry after mixing, let it stand for a while, observe the size and density of the sludge flocs, as well as the clarity of the supernatant. The flocs are large and dense, and the clearer the supernatant, the more appropriate the dosage is. Finally, record in detail the dosage and corresponding effects of each beaker, compare and analyze to find the optimal dosage.
2. On site pilot "verification"
-After finding the approximate range of dosage in the laboratory trial, it is necessary to bring it to the site for pilot testing. Because there are still differences between laboratory conditions and actual on-site conditions, such as temperature, water quality fluctuations, and so on.
-Select a dehydration equipment on site and set different dosages for trial operation according to the dosage range obtained from the small-scale test. During operation, closely monitor the operation of the dehydration equipment, such as the clarity of the separation liquid in the centrifugal dewatering machine, the moisture content of the filter cake in the belt filter press, and the filtration time of the plate and frame filter press.
-Based on the results of on-site trial operation, fine tune the dosage until the optimal dosage that can ensure dehydration effect and control costs is found.
3. Online monitoring "dynamic adjustment"
-Many sewage treatment plants now use online monitoring equipment, which can monitor the concentration of sludge, the dosage of PAM solution, and the moisture content of dehydrated sludge in real time.
-Based on these data and the pre-set control program, the system can automatically adjust the dosage of PAM. For example, if the sludge concentration suddenly increases, the system will automatically increase the amount of PAM added; If the moisture content of the dehydrated sludge exceeds the standard, the dosage will be adjusted accordingly. However, online monitoring equipment requires regular calibration and maintenance, otherwise the data will be inaccurate and the adjusted dosage will be unreliable.
Small Tips for Daily Operations
1. The dissolution of PAM is crucial: when dissolving PAM, clean tap water must be used, and the water temperature should be controlled at 20-40 ℃. If the water temperature is too low, the dissolution will be slow, and if it is too high, it may damage the molecular structure of PAM. In the dissolution process, the stirring speed should not be too fast to prevent excessive foam. Generally, the solution should be stirred for 40-60 minutes until it becomes uniform and transparent, free of lumps and flocs.
2. Regularly review the dosage: The properties of sludge may change with factors such as season and inlet water quality. So we can't just ignore a certain dosage, we need to do a small trial regularly (such as every week or every half month) to see if we still need to adjust the dosage.
3. Try different brands of PAM: There are many brands and models of PAM in the market, and the quality and performance of products produced by different manufacturers may vary. You can find several reliable brands and conduct comparative tests in the laboratory and on-site to select the product with the highest cost-effectiveness.
Determining the dosage of PAM for sludge dewatering is a task that requires patience and experience. We need to understand the "temperament" of sludge and PAM, and combine it with the actual treatment process. Through continuous experimentation and adjustment, we can find the most suitable dosage, which can not only treat the sludge properly but also save a lot of costs!
