SOG-PQQ-GW-Fontaine Vinery Emptying Point Refurbishment

Properties within Guernsey which produce waste water but do not have a permanent sewer connection use cess pits to store their waste water/sewage instead.

Cess pits are routinely emptied by sewage tankers, and the collected load transported to one of several locations where the tankers can connect to the main sewer and offload. These locations are referred to as [sewage tanker] emptying points. Guernsey Water is in the process of rationalising the network of emptying points.

As part of the rationalisation, Guernsey Water would like to refurbish the Fontaine Vinery emptying point. It drains to a pumping station which is in turn connected to the gravity network. There are 4 domestic properties which also drain through the emptying point into the pumping station. The pumping station experiences a very high frequency of pump chokes due to ragging which causes the emptying point to shut down whilst these are cleared.

The pumps have hardened impellers which should break down the rags, and it was previously assumed that the tankers must be collecting a much higher volume of rags than allowed for. A screening system was trialled at another emptying point but instead of rags has predominantly removed grit and aggregate.

This grit and aggregate is believed to be concrete from the cess pits which has decayed due to H2S attack from the sewage turning septic prior to collection. The tankers are now thought to be inadvertently collecting this along with the sewage and discharging it to the network, which is damaging the cutting surfaces of hardened impellers and preventing the rags being broken down as they should be.

It is thus intended to install a grit tank to capture this before it can come into contact with the pump impellers and damage their cutting surfaces, leading to a rag choke, which shuts down of the station and emptying point whilst this is cleared.

At the same time the tanker connections which are currently recessed will be swapped for above ground concrete tanker blocks as at other emptying points, and some minor repairs/maintenance to the area in general.

The works involve;
• installation of a pre-fabricated 2m diameter grit chamber below ground. Excavation required is approximately 3.5m depth.
• Re-laying approximately 30m (net) of sewer pipework with diameters ranging from 160 to 250mm
• Cast in situ or prefabrication of 3 tanker connection blocks to specified dimensions.
• Reconstruction/Resurfacing of approximately 90m2 of access road following the installation.