Force Main Termination 1

[vmirat]

We have an existing sanitary force main that I'm evaluating. The 8" main terminates at a manhole with the crown of the force main at the same elevation as the crown of the 8" gravity line leaving the manhole. This is per City code.

I have seen other municipalities require that the crown of the force main be 2 inches below the invert of the gravity line so that the force main is always full. I like this design versus what our local city code requires.

Comments?

 

[amlinerrichard]

With the force main full you won't get quite as much splashing in the manhole. This will reduce the aroma coming from that manhole. Also will develop less sulfide gases.

 

[nackra]

length of forcemain (long detention time) will impact the "aroma"

 

[cvg]

H2S is generated where there is a lack of oxygen, so limiting oxygen is not necessarily the best approach. Also, unless the force main profile is rising, than the force main is not entirely full.

 

[RSD90]

Having the crown of the force main below the invert of the outgoing sewer shall have the following issues:

1. Sewage could turn turn septic if it stands within the main/manhole idle (i.e. which will happen during the lean hours). Moreover it shall also start depositing the particulate matter present within it, which in future shall affect the carrying capacity of the force main.

2. In case there is a pump failure (due to power outage or any unforeseen reason) the sanitary waste within the force main + the sanitary waste within the manhole shall flow back causing a water hammer (i.e. upsurge). This pressure if high can cause a lot of damage.

Now

Having the crown of the force main in line or slightly above the outgoing gravity sewer main shall benefit in the following way:

1. With no sanitary sewage within the force main there won't be any septic conditions and depositions too. Hence benefiting the force main over a long duration.The flow that is transferred to the manhole shall immediately pass away as compared to the condition were we shall always have a 8"+2" standing the sanitary waste within the manhole. A standing flow shall cause nuisance odour, septic conditions, particulate depositions thus creating need for frequent maintenance.

2. In case of an abrupt pumping failure the flow shall be low and consequently the water hammer (i.e. upsurge) pressure shall be low causing no damage.

Hope you find this satisfactory enough!

RSD
Water & Wastewater Engineer