031 paper on sludge fluidity presented in NCEVT'10-11

Emerging Trends in Civil Engineering, December, 04 &05, 2010, PIET, Limda

NCEVT’10 Devang Shah 193

Effect of Silt on Fluidity of Digested Sludge of Sewage Treatment Plant

Devang A. Shah Asst. Professor, Civil Engineering Department, Parul Institute of Engineering and Technology

Limda [email protected]

ABSTRACT Depending on the frequency of sludge withdrawal the consistency of the digested sludge withdrawn from the low rate digester is expected to be in the range of 4- 6 % and for high rate digester is expected to be in the range of 6-8%. Abnormal increase was observed after monsoon period in consistency of digested sludge and digested sludge has lost its fluidity. In this paper, the factor affecting this loss of fluidity of sludge is reviewed and represented. Key Words: Sludge, Silt, Sewage INTRODUCTION In the Sewage treatment plant under study the process description is as follows. Sewage collected from the various auxiliary pumping stations (APS) shall be pumped to the inlet Chamber to the treatment plant. The sewage shall flow through the Mechanical bar screen whereby the floating solids, plastic bags, rags, etc. shall be removed by the mechanism. The material removed from the sewage shall fall in to trolley placed on ground. After screening, grit particles shall be separated and removed in Mechanical grit detrittor. The sewage then flows to Primary Settling tank for settling and removal of suspended solids from the sewage. The solids settled in the PST are scrapped and brought to the center through the mechanical sludge scrapper. The settled sludge is removed from the center, collected in primary sludge sump and pumped to the Thickener. After primary treatment, settled sewage from the PST overflows to the aeration tank through overflow launder. The aeration tank is the key unit of the treatment plant and is divided in to two compartments for operational flexibility. Each tank is provided with fixed surface aerators for supply of atmospheric Oxygen to the sewage. For biological treatment micro-organisms are cultured in these tanks. The micro organisms developed in the aeration tank, in presence of

atmospheric oxygen provided by the surface aerators, consume the organic matter (BOD) and convert them in to gases and sludge solids. After the aeration the sewage along with the biological solids flow to the secondary settling tank where the bio-solids are settled out and removed from the bottom in the form of biological sludge. The sludge contains active micro-organism and a part of it is recycled to the Aeration tank, while the excess sludge is diverted to the Thickener. The treated sewage, overflowing from the top launder, is disposed off in the nearby natural drain. The primary sludge from the PST and the secondary sludge from the SST pumped to the thickener shall be thickened in this unit and shall be withdrawn from the bottom. The thickened sludge shall be pumped to the Digester for its digestion and stabilization. The overflow from the thickener is only partly treated and is sent back to the nearby pumping station for the further treatment. Two digesters are provided in the process to run in parallel. Each digester is provided with four mixers for the mixing of sludge and separation of gas. During the process of digestion part of sludge is converted in to gases (mainly Methane) and inert solids, thus reducing the volume of sludge, which can be easily disposed off, and producing Bio Gas. The supernatant overflow from the Digester shall go to the Aeration tank for further treatment.



The digested sludge, free from foul smell is removed from the bottom and dried on sludge drying beds. The sludge cakes can be lifted when dry, and used / sold as manure. By pass arrangement has been provided for all the strategic units so that maintenance work can be easily taken up with out stopping the entire treatment plant. BASIC DATA To study the plant performance the basic data taken during normal operation of plant is represented here in Table 1 to 5. In the monsoon period generally silt comes with the surface runoff flowing through the drains. Due to dilution generally the COD and BOD of sewage after rain is very low and this water is bypassed directly without treatment. But some times when only small amount of rainfall is there then dilution is not so high and waste water is required to be treated. This flow when taken into plant all the silt particles will get settled in PST and removed with sludge and pumped to sludge thickener. During that particular monsoon the amount of silt observed was very high. Later it was found that there was application of dirty water flow from small pond in particular area due to municipal corporation works going on. This water was carrying lot of silt with it and number of times the sludge withdrawal line of primary settling tank was observed to be chocked up due to silt. Still by applying back pressure with water hose and motor the line was cleared and plant continued to run. But the great mistake happened was that to take this silt laded sludge to thickener and from there to digester. While this silt was entering in plant inflow not only primary settling tank sludge withdrawal line was observed to be chocked but while it was pumped to thickener the sludge withdrawal line of thickener was also observed to be chocked. Unluckily in spite of all this the sludge containing excessive silt content was pumped and fed to digesters. Now slowly this silt has started to affect the quality of digested sludge. The digested sludge is easily flowable slurry of consistency 6-7% normally. But due to silt which was getting accumulated at bottom the consistency started to increase and it increased so high that instead of fluid sludge lumps are formed. The periodic increase in consistency of sludge is represented here in tabular form in Table 6. Table 6: Digester sludge consistency data

The lumps has caused hindrance in flow in 50m long

sludge withdrawal line from digester and condition has aroused like there was no sludge flow at all even if sludge withdrawal valve is fully open. By applying back pressure with water hose some flow was obtained and lumps with very thick sludge has emerged from sludge withdrawal pipe. CONCLUSION These lumps were having consistency of 26.63% and organic content was 35.8% while inorganic content was 64.2%. This analysis and physical inspection has shown that these lumps were formed due to presence of silt. So, from this it is concluded that it is not advisable to take sludge with very high silt content in anaerobic digesters. The most trickiest part is the detection of high amount of silt, because in monsoon generally due to dilution the SS content of sewage is reduce which may be compensated by increased amount of silt and total SS content do not rise significantly, and hence just by reading the experimental data no one can judge that a serious problem is there in pipe for him. The detection of silt is other wise very easy by taking

Digester sludge consistency

Date Digester A

% Digester B

%

27-Jun-06 7.8

30-Jun-06 7.9

3-Jul-06 7.78 7.26

6-Jul-06 8

14-Jul-06 8.8

17-Jul-06 8.29 7.5

21-Jul-06 8.2 7.3

25-Jul-06 8.2 7.1

28-Jul-06 7.1

4-Aug-06 13 9.1

5-Aug-06 12.98

19-Aug-06 11.62 8.56

21-Aug-06 16.59 13.68

22-Aug-06 16 14.32

1-Sep-06 16.21 16.44

2-Sep-06 14.81 20.75

5-Sep-06 14.26 20.02

9-Sep-06 18.44 14.95

14-Sep-06 15 14.51



the sample in a beaker and let it settle down for half an hour and then by inspecting the sludge collected at bottom by physical and chemical inspection presence of silt can be determined and if sludge is containing higher amount of silt than it is better to take this sludge directly to sludge drying bed or for disposal instead of feeding the same to the digesters. Regarding chocking of digester sludge withdrawal pipe the same was cleared by lot of hard work in form of applying water from bottom and taking out lumps with flow of that water. After long efforts all the lumps were removed with flow and again fluidity of sludge was established but the period lapsed during was not less than a nightmare for the operators of plant. If digester sludge withdrawal pipe is chocked than it will stop the proper functioning of digester and it will become full of sludge as no withdrawal is possible. Hence it is very important not to feed sludge with excessive silt content to digesters.

REFERENCES

1. CPHEEO MANUAL FOR SEWAGE 2. Waste Water Engineering Treatment and

Reuse- Metcalf and Eddy



Table 1: Analysis Report for Plant inflow and outflow



Table 2 : Aeration Tank data

DISSOLVED OXYGEN

DATE AERATION TANK-A

MG/L

AEARATION TANK-B

MG/L

MLSS MG/L

MLVSS MG/L

SVI ML/G

9-Mar-06 1.8 2.2 3001 1948 30 16-Mar-06 2 1.9 3090 1915 48.5 23-Mar-06 2.4 2.2 3500 2100 35.85 2-Apr-06 2.4 1.9 3050 1875 42.63

Table 3 : Digester sludge data

DATE pH VFA MG/L

N GM/KG

9-Mar-06 6.9 1064 5.6 16-Mar-06 7.4 1314 7.2 23-Mar-06 6.8 1069 6.8 2-Apr-06 7.4 1188 5.5

Table 4 : Sludge Consistency data

DATE

PST Under flow

Consistency %

PST Over flow Consistency

%

Thickener Under flow Consistency

%

Thickener over flow

consistency %

Digester Under flow Consistency

%

Digester over flow

consistency %

9-Mar-06 2.2 0.039 6.2 0.07 7.9 1.2 16-Mar-06 2.9 0.041 6.8 0.06 8.2 1.8 23-Mar-06 3.2 0.029 7.2 0.072 8.6 1.4 2-Apr-06 1.9 0.035 7.6 0.056 7.4 1.3

Table 5 : Digester sludge content data DATE : 2-Mar-06 INORGANIC CONTENT : 63% ORGANIC CONTENT : 37%



Documents

031 paper on sludge fluidity presented in NCEVT'10-11