Alum Dosing

Creation of an Alum Dose Curve

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. Experimental data shows that as temperature increases, less alum is needed. This data is displayed in Table 1.

Table 1. Rule of thumb data used by Cornell University's Water Treatment Plant Operators.

A log relationship equation (Y = A + B*log(NTU)) was used to automate 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.

Observations of Flocculation at Incremental Alum Doses

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 used:

*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 there 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.

*At an alum dose of 20 mg/L, there initially appeared to be a 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 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 appeared earlier in the tank, about a third of the way down the first section. They were still small at this point but the improvement over the end of the first section and the middle of the second section was rapid. Most of the particles were in floc and there were not as many particles in between flocs.

*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 appeared at a dose of zero. Throughout the tank there were many medium to small sized flocs. 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 was expected. Experimenters had predicted the same type of floc distrubution as had been observed earlier when the tube settlers clogged. Overdosing alum appears to cause it to lose its effectiveness.

Alum Dose Calculations

For the majority of testing, alum dose was set by the aforementioned log relationship equation (Y = A + B*log(NTU)). After A was adjusted from 15 to 10, this approach was effective for the low turbidities that the flocculator experienced this summer. At other times of year when the raw water turbidity is higher, the effectiveness of this relationship can be tested in a higher turbidity range. Through use of that equation, 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.

Optimal Alum Dose Testing

Because the pilot plant takes water directly from the stream, environmental conditions change all the time and affect the incoming turbidity to the plant as well as the chemical composition of the particles causing turbidity. It is therefore necessary to determine the best alum dose for each day of testing to ensure the formation of good flocs. This requirement was implemented in the following way:

The Process Controller was used under the "Alum Not Increment" setting, which starts the Alum Dose at 0 mg/L, and increments at 5 mg/L until it reaches a maximum of 40 mg/L. Each of the alum doses ran for a 30-minute period, or for about 3 times the residence time of the tank. After the completion of the test, data processing was performed to select the data from the last 10 minutes of each individual alum increment. The first 20 minutes of data were rejected because the residence time in the flocculator was 10 minutes, and the residence time in the tube settler was also about 10 minutes. Therefore, in order to get readings from the final turbidimeter that were representative of the alum dose that we were testing, we discarded the first 20 minutes of data at each alum dose. The outgoing turbidity was then analyzed for each increment, and the alum dose achieving the lowest turbidity was selected for the second part of the experiment.

However, one rule of thumb that may be used under lower temperatures and incoming tubidity levels can be to rely on the PAC (Polyaluminum Chloride) dosage use by the Cornell Water Filtration Plant Plant Operators. According to the active ingredients for the alum and PAC along with the equivalent ratios given for both coagulants, 1 ppm of PAC is approximately equal to 0.182 mg/L of alum.