Desilting of Sewers

The desilting operation of sewers proceeds from manhole to manhole. The process starts from the sewer line located most down stream. The lower end of the sewer line is blocked with the help of sand bags. The blocking of sewer with sand bags requires entry of a sewer man into the manhole. Prior to the entry of sewer man into the manhole, manhole covers from both ends of the sewer line to be cleaned are removed. The sewer line is then flushed with air by compressors. The flushing of air removes the harmful gases from sewer line and makes it safe for human entry. However, in order to confirm that sewer line and manholes are free from all harmful gases multi gas detector is lowered in the manhole. The multi gas detector measures the concentration of harmful gases i.e., H2S and CO2, CO as well the concentration of oxygen in the manhole and sewer line. The instrument transmits concentration of harmful gases and oxygen out side the manhole through a cable, which can be seen on the screen held by supervisor of the operation.

After the supervisor of the de silting operation has made sure that manhole is safe for human entry, sewer man wearing gloves, breathing apparatus, helmet with light and water proof shoes with oxygen cylinder on his back and rope tied to his belt enters the manhole. The other end of the rope is attached to a man winch. This arrangement is made to lift the sewer man out of the manhole in case of occurrence of any untoward incident. The sewer man fixes sand bags on the lower end of the sewer line and makes sure it has become leak proof and comes out of the manhole.

The sewer man then enters the upstream manhole to fix the pipe of the jetting machine to the sewer line and comes out. Supervisor of the de silting operation then orders to start the jetting machine. The jet of water erodes the solidified silt with its thrust. Water requirement of the jetting machine is fulfilled by the water tanker, which brings water from the nearest tube well. Jetting of the solidified silt and its removal proceed simultaneously. Sludge suckers are used to withdraw silt from the sewer line and manholes.

In the sludge sucker silt being heavier than water readily settles at the bottom and supernatant water is collected at the top. After the sludge sucker is filled, the supernatant water, which is free from silt is drained in the nearby drainage channel and sludge sucker is dispatched to the landfill site for silt disposal.

The de silting operation will proceed between 10.0 pm at night to 6.0 am in the morning. This time is selected for two reasons:

• Flow of water in the sewers is almost zero
• Traffic flow on the roads is at its lowest

De silting of drainage channels

In the drainage channels the mechanism of de silting is simpler than in sewers. The reason being, the channels offer an open environment whereas in sewers environment is closed. The probability of accidents and health and safety requirements are lesser. Instead of using jetting machines and sludge suckers the process involves the use of excavators, clam shells and dumper trucks.

The process starts by setting up the excavator/clam shell at site. The choice of machinery is made on the basis of site requirement, channel width and the volume of the silt to be removed. The silt is removed from the channel with the help of bucket attached to the machine. The silt collected in the bucket is unloaded in the dump trucks, which takes the silt for final disposal to the solid waste landfill site. In order to ensure that silt with least water contents is thrown into the dump trucks, the silt filled bucket hangs over the drainage channel for few minutes so that maximum water contents drop back into the channel. This step is necessary to avoid the splashing of water on the roads from the dump trucks that makes the road slippery causing inconvenience to the passer bays.

Health and safety requirements are gloves, safety shoes and helmets. First aid boxes should be available on site.

Methodology to be Applied for the Project

Cleaning methodology of sewers and drains as explained in Section 3.2.1 will be applied under the project except the practice of labor entering manholes will discontinue. The activities where labor is presently employed in de silting operation are blocking of one end of sewers and the fixing of jetting machine nozzle at the other end. Under the project it will now be achieved through mechanical means. The de silting process proposed for the project can be seen at following

De silting Plan

De silting plan is mainly a distribution of machinery among different project activities. De silting plan has been prepared by assessing the capacity each machinery for removing silt in a day. The silt removal capacity has been based on the working hours, the velocity of silt transport vehicle and the distance of sewage zone from the solid waste dumping site. On the basis of this capacity actual number of days and years required to clean sewers and drains has been worked out. The analysis show that all the sewers and drains included in the project can be cleaned within a period of three years. Where the new machinery has been found insufficient to meet the target, existing machinery available with WASA has been spared and made available for the project. In case of the de silting of drains, 10 dump trucks to be supplied by the Government of Japan have been found insufficient to meet the target against the requirement of 20 dump trucks. The short fall will be met by inducting a fleet of 10 dump trucks in the project, to be supplied to WASA by the Government of Punjab in December 2005. The expected date of the commencement of the project is April

For ease of operation, sewer cleaning of Sewage Zones A and B have been assigned to one Sub Division, whereas for sewage zone G and H1 separate SDO's will be posted for undertaking sewer cleaning operation. The cleaning of drains of the whole project area has also been assigned to one Sub Division. Such arrangement has been made to maintain uniform distribution of workload in the subdivisions. The details of de silting plan are as