TITLE:

Sludge Settleability Tests and Determination of the Mixed Liquor Suspended Solids and the Mixed Liquor Volatile Suspended Solids of activated sludge

AIMS:

  • To determine the operating efficiencies of various sewage treatment plants

INTRODUCTION:

Treatment plant operators should keep a close watch on settling characteristics because a trend towards poor settling can be the forerunner of a badly upset plant. The settleability of activated sludge is most often described as The Settled Volume (sv).. The sv is measured by placing one litre of a well-mixed sample of activated sludge into an ‘ Imhoff ‘ cone and allowing it to settle for 30 minutes. The volume occupied by the settled sludge after 30 minutes is the SV. The success of the activated sludge system depends on many factors. Of critical importance is the separation of microorganisms in the final clarifier. The microorganisms in this system are sometimes very difficult to settle out, and the sludge is said to be bulking. Often this condition is characterised by a biomass comprised almost totally of filamentous organisms that form a kind of a lattice structure with the filaments and refuse to settle.

The amount of solids present in the activated sludge has a major influence on the degree of settling. This is take into account by determining the Sludge Volume Index (SVI) which is defined as the volume in millimeters occupied by one gram of sludge after 30 minutes settling. SVI values below 100 are usually considered acceptable with values greater than 200 defined as badly bulking sludges. When the sludge does not settle the return-activated sludge becomes thin and thus the concentration of microorganisms in the aeration tank drops. This results in higher F/M ratio and a reduced BOD removal efficiency

MAERIALS & METHODS:

As per manual

RESULTS:

Calculation of results

Settled Volume (SV) = 775

Settled Volume Index (SVI) = SV (ml) * 1000 / mlss (mg/l)

(SVI) = 775 * 1000 / 8130

(SVI) = 95

Table 1 MLSS (mg/l)

filter no. / Before filtration / after / mls sample used
1 / 0.2002 / 0.3628 / 20

.formula

MLSS mg/l = gross wt. - tare wt. *1000 / ml of sample

MLSS = 362.8 - 200.2 *1000 / 20

MLSS = 8130 mg/l

Table 2 MLVSS

crucible no. / Wt. Cruc + fiter paper
pre ignition / Wt. Cruc. + filter paper post ignition / ml of sample
1 / 46.5854 / 39.4914 / 20

MLVSS mg/l = wt of crucible + filter pre ignition) - (wt. of crucible + filter post) *1000 / ml of sample

(46.5854) - (39.4914) *1000 / 20

MLVSS = 355 mg/l

Class Results

Table 3

Group No. / Sludge
Location / MLSS
mg/l / MLVSS
mg/l / SV
ml / SVI
ml
1A / Collooney / 8130 / 355 / 775 / 95
1B / Strandhill / 3306 / --- / 860 / 260
2A / Strandhill / 3610 / 225 / 830 / 229
2B / Strandhill / 3510 / 329 / 830 / 236
3A / Collooney / 8805 / 400 / 750 / 85
3B / Collooney / 8368 / 480 / 750 / 89
4A / Collooney / 8550 / 230 / 775 / 91
4B / Collooney / 8284 / 350 / 775 / 94
5A / Strandhill / 2356 / 175 / 800 / 340
5B / Collooney / 8147 / --- / 800 / 98
6A / Collooney / 7636 / 400 / 750 / 98
6B / Strandhill / 3495 / 256 / 800 / 229
7A / Collooney / 8300 / 404 / 825 / 99
7B / Grange / 580 / 125 / 125 / 215

DISCUSSION:

Sludge settleability is dependant on the microbiological, biochemical and physio-chemical properties of the sludge. Activated sludge will settle well if the SVI <100, whereas an SVI <150 indicates settling problems and possible bulking. The sludge volume index obtained for Collooney sewage treatment plant during this practical was 95 which would indicate that the sludge is settling well. Looking at the class results in Table 3 it can be seen that the SVI for all of the Collooney sludges tested was below 100. As can be seen the results obtained on performing the mlss and mlvss showed variance, with result obtained for the mlss being lower. The mlss is the concentration of suspended solids in the aeration tank. It is a crude measurement of the biomass withen the aeration tank. This is measured in the same fashion as suspended solids. For conventional activated sludge units the mlss ranges from 1500 - 3500mg/l., but for high rate systems it can be as high as 8000 mg/l. The mlss of the Collooney sludge ranged from 7636 mg/l to 8805 mg/l.. The mlss concentration rate can be altered or controlled by altering the sludge wastage rate. Usually the higher the mlss in a system the greater is its efficiency as the biomass has a greater capacity to utilise food available. The mlss may contain some inorganic matter so it is estimated that the organic fraction of the sludge is measurable by the combustion of the matter present in the mlss and this is known as mlvss. The proportion of active microorganisms in mlvss depends on the operating conditions as well as the amount of volatile solids in the wastewater. The mlvss of the Collooney sludge sample ranged from 350 to 480 mg/l. From the results obtained on the Collooney sludge it can be assumed that Collooney sewage treatment plant is working well.

The other sludges used in this practical were from Strandhill and Grange sewage treatment plants. From the class results in Table 3 it can be seen that the sludge in Strandhill and Grange is not settling well, ie the SVI’s for the Strandhill sludge were in excess of 200, and 215 for the Grange sludge. The sv of the Grange sludge was a low 125, which would indicate that the sludge is not settling well. The mlss was also very low being only 580 mg/l. A value as low as this would indicate that Grange sewage treatment plant is operating inefficiently.

There are some points of interest in the class results. Group 5b who carried out tests on the Collooney sludge reported a mlss value of 4075 mg/l and a svi value of 196ml when the results were first presented on the board. However when presented again some weeks later the results had changed somewhat, to a mlss value of 8147 mg/l and a svi value of 98 ml. It can be seen that the mlss doubled while the svi halved. A situation such as this would almost certainly be a concern to any sewage treatment plant operator!

CONCLUSION:

From the class results it can be assumed that Collooney sewage treatment plant is operating most efficiently.