Side Water Depth Considerations (Part 2) by William H. Boyle, P.E.

Solids Can Hinder Performance

If there is an appreciable amount of solids in the clarification zone, the performance of the clarifier will be inadequate.

The zones beneath the clarification zone are affected by the amount and makeup of the solids coming in to the clarifier. As the mixed liquor comes into the clarifier and settles, the associated water flows from the mixed liquor mass towards the effluent, thereby depositing the solids towards the bottom zones. The incoming mixed liquor will flow over the settled sludge if there is a noticeable difference between their densities. With low MLSS, a noticeable difference would occur at the bottom of the clarifier where the ultimate concentration appears below the compression zone. With higher MLSS, the hindered zone and transition zone become more of a governing factor on the flow pattern of the mixed liquor.

Let’s look at five cases of possible theoretical bench test results for further understanding. 

The dimension-less parameter Sludge Volume Index (SVI) will be used for the development of this rationale. Sludge Volume Index is the volume in milliliters occupied by one gram of activated sludge after settling for 30 minutes, expressed as:

SVI = ml of settled mixed liquor after 30 min / ppm SS in mixed liquor x 1000

SVI = ml @ 30 min / MLSS x 1000

It is readily agreed upon that as the SVI increases over 100 poorer settling characteristics of the mixed liquor will be realized. Looking at a typical SVI of 100 and relating this to various MLSS concentrations, the following can be developed: 

For SVI = 100	MLSS	ml @ 30 min.	% of depth
Case 1	2000	200	20
Case 2	3000	300	30
Case 3	4000	400	40
Case 4	5000	500	50
Case 5	6000	600	60

For every 1,000 ppm of MLSS increase, the ml settled solids @ 30 min. increased by 100. Percentage of depth is based on the milliliters of solids settling in a 1,000 ml graduated cylinder.

It is obvious by looking at the five cases the clarification zone in Case 1 is larger than in Case 5.  In an actual plant, if a given clarifier with a given depth was designed and operated at the condition found in Case 5, it could be anticipated that the effluent quality would suffer due to the lack of available clarification zone. If this was the case, then the high MLSS would require a greater basin depth to maintain an adequate clarified water zone because the incoming mixed liquor solids contact more solids in the hindered and transition zones.

The design engineer normally does not have the luxury of running actual bench tests for the plant that is about to be designed. Since this is often the case, then the following might be a useful procedure to use when determining the side water depth for the final clarifier, keeping in mind the above discussion. An investigation into an existing clarifier’s performance could also use this tool as to evaluate performance.

Depth Percentage

Based on the premise that a clarifier handling 2,000 MLSS with an SVI of 100 performs adequately at a 10-foot side water depth, a percentage of depth to MLSS versus mL ratio can be determined. The above shows the percentage of depth at the various MLSS. Note that for every 1000 ppm MLSS increase, the percentage of depth, based on the milliliters of solids settling in a 1,000 ml graduated cylinder, increases by 10%. Thus, a rational conclusion would be: as the MLSS increases by one thousand ppm, the side water depth should increase by one foot. This is to maintain the same clear water depth above the sludge blanket.

By using the data, the five cases depict the change in MLSS and indicate that although the initial settling rate might be somewhat the same; the zones below the clarification zone become deeper, reducing the depth of the clarification zone. An extreme case would occur if the clarifier was operating with the sludge blanket level at or near the basin water surface causing gross carryover of solids.

If the one foot increase in side water depth for every 1,000 increase of MLSS is used as a guideline, then any mixed liquor below 100 SVI should be acceptable.

High SVIs?

There is certainly a limitation with the use of the above SVI approach in evaluating side water depth in relation to the effluent quality. As stated above, an SVI over 100 results in poorer settling characteristics of the mixed liquor. Looking at sludge with an SVI of 200 and an MLSS of 5,000 the percentage of depth would be 100% (200 SVI * 5000 MLSS/1000 = 1000 ml @ 30 min = 100% depth). Since this is somewhat unrealistic, the guidelines of using SVI for determining side water depth would be in question for high SVIs. However, it might be an indication of why there is a poor effluent quality with high SVI sludges. In this instance, the high SVI relationship to effluent quality from the final clarifier should be looked at relative to the overflow rate, inlet structure, sludge removal mechanism, and/or mass loading.

The above are parameters that need field data and/or field observations to verify. The state point analysis is another useful tool to get a “feel” for what is happening inside of a circular clarifier.