Alum Dosing

Alum Dose

Alum dosing has been an important variable throughout flocculation history. It is considered an art form and is generally picked up The proper dose of alum to optimize flocculation for a given set of plant conditions is difficult to determine. Alum dosing is a skill which is generally acquired through practice and experience. At the Cornell Water treatment plant the operators rely on past data and a streaming current director to establish their alum dosing. They also rely on some rules of thumb that are affected by the temperature and the turbidity of the water. The experimental Experimental data shows that as temperature increases, less alum is needed. This data is displayed in Table 1.

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In an attempt to automate and decrease the mystery behind alum dosing the A log relationship equation (Y = A + B*log(NTU)) was used to test its impact on flocculation in the pilot plant flocculatorautomate alum dosing, and effects on the flocculator were observed. After the value of A was lowered and the tube settlers stopped clogging there appeared to be good floc formation and clean water being produced in the flocculator. The alum dosing was sufficient so that by the end of the second section the turbidity was usually around 1 NTU and was almost always below 2 NTU. The raw water turbidity coming into the flocculator stayed between 2 and 6 NTU during most tests that were run. While the turbidity is low, there is good floc formation and allows clean water to be produced by AguaClara technology.

Alum dosing was also investigated by watching floc formation in the flocculator at different alum doses. This was done in an attempt to note if it was possible to visually discern when the alum dose needed to be changed. Doses of 0, 5, 20, and 50 mg/L were notedused.

Upon observing 0 At an alum dose of 0 mg/L, it was very clear that there wasn't any improvement through the tank. In fact the whole way through the flocculator it looked as though there were tiny particles floating along with big particles and this never changed.

When observing 5 mg/L the difference was hard to discernthere was little discernible difference from the zero dose. It appeared that there was some improvement through the tank. The last two sections didn't appear to have as many small particles in between some of the larger particles were but the flocs that were seen appeared to be smaller than those observed previously.

When I raised the At an alum dose to of 20 mg/L, there initially appeared to be a rush pulse of flocs at the beginning of the flocculation tank that moved up the first section of the tank. It is unclear if this was due to the introduction of the higher alum dose. After the initial rush pulse of flocs, the water entering the tank did not appear as turbid as the raw water entering the tank without any alum. The floc formation appears appeared earlier in the tank, after about a third of the way down the first section. They are were still small at this point but the improvement over the end of the first section and the middle of the second section is was rapid. Most of the particles are were in floc and there are were not as many particles in between flocs.

When I finally increased the dose to At an alum dose of 50 mg/L the water in the first section appeared to have the same distribution of small particles in between larger particles as the raw water when there was no alum added.. Through the tank it appeared that there lots of appeared at a dose of zero. Throughout the tank there were many medium to small sized flocs, almost as though there were more medium sized flocs and not any large flocs. It does not look like the flocs are large. They are small and never get bigger throughout the tank. There are a few rare . The flocs did not increase in size as they traveled through the flocculation tank. There were only occasional flocs that could be considered large. This is contrary to what I had expected to see, I thought I would see was expected. Experimenters had predicted the same type of floc distrubution as had been observed earlier when the tube settlers were cloggingclogged. Overdoing the Overdosing alum dosing appears to have negated the effect of the alum, by creating the reverse charge on the particles which again had a repelling affectappears to cause it to lose its effectiveness.

Alum Dose Calculations

For the majority of testing, alum dose was set by equation 18. After the change of A from 15 to 10 was made this approach was effective for the low turbidities that the flocculator experienced this summer. Hopefully in the future the raw water turbidity will change enabling testing of higher turbidities. Through use of equation 18, observing the floc tank and conversations with the operators at the water treatment plant it has become apparent that there is still a lot of research that needs to be done regarding alum dose. Observing the floc tank was helpful in being able to identify different kinds of floc and what different alum doses looked like in the water entering the water treatment plant. The water treatment plant has now switched to a different coagulant but if they had to go back to alum they said they would use past experience and alum doses as well as jar tests to set their doses. This suggests that for each water treatment plant an equation, formula or at least a rule of thumb could be developed off of past water treatment for future dosing. If this formula would be translatable to other water treatment plants and different water types is uncertain. The run increment alum dose test should help to shed light on alum dosing as it allows the alum dose to be changed while at a relatively constant raw water turbidity. Hopefully the data from this test will show either an optimum dose or a small range of optimal doses for specific settled water turbidity.

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