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 from 15 mg/L to 10 mg/L 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 of the flocculator 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.

Conclusions

For the majority of the testing done this summer, 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 could be tested in a higher turbidity range. Through use of this equation, observations of 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 tank. The water treatment plant has now switched to a different coagulant (PAC) 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 from past water treatment for future dosing. Due to variations from plant to plant in influent water, it is improbable that this formula would be useful at other water treatment plants. The run increment alum dose test helped to shed light on alum dosing as it allows the alum dose to be changed while at a relatively constant raw water turbidity. The data from this type of test should show either an optimum dose or a small range of optimal doses for specific settled water turbidity.