When we started installing monitoring systems, one of the most interesting things we noted was how many times some of the valves were firing.
In a gravity sewer you really have no idea how much sewage is entering the system from each connection point. In vacuum systems if you really wanted to know, you could put a digital counter on the valve top to record how many times the valve operated. As we know that the valve mainly fires each time there is 10 gallons (40 liters) of sewage in the pit then it is not too hard to determine how much flow is entering the pit. And as there is usually 2-4 houses connected to a pit you can usually determine where any abnormal flows are coming from.
When a commercial user is attached to your vacuum sewer network whether that be a school, a shopping center, a factory, service station, restaurant or hotel, council usually enters into an agreement with that customer about what flows can be discharged into the sewer. How then can you make sure that the agreement is being kept.
A vacuum valve will typically fire about 100 times per day in the USA and about 65 per day outside of the USA where flow rates are lower. We do know that the valves can handle higher flows than this with a 3 gallon per minute (0.25 l/s) rating in the US and elsewhere. But we always need to be careful when we design systems which are seeing higher flows than normal residential areas so that not only can the vacuum mains be sized correctly, but so that we can ensure that the vacuum recovery to the pits is taken into account. In double pits this is especially true. We also have to take into account emergency storage in the collection pit. 4-6 hours is the normal standard but if there is a high flow a larger collection pit may be required to get the storage that is needed.
On a vacuum system where we have installed a number of monitors into high flow commercial pits the utility manager was really suprised by the number of times each valve was firing each day.
So imagine our surprise when the Director saw that one Hotel was putting in enough sewage for 1,762 valve firings on the day. (not a day when it was raining, so this was not infiltration). Across the road from the hotel a well know local restaurant was using its pump system to pump sewage into its vacuum collection pit and it was firing 586 times in a day. This is 23,480 gallons (90,000 liters) through just two vacuum valves over the road from each other, every day.
For quite some time this system had been suffering from flooding of the vacuum lines, loss of vacuum at the end of the line and high level alarms at pits further downstream. Now, due to the monitoring system, the Director could identify exactly why it was happening and could have a conversation with the commercial owners about their flow rates.
We have seen high flow rates come from schools at times. It could be due to automatic flushing of the toilets or dumping water from the swimming pool. These types of flows can be corrected and are good to know about.
High flows entering the system can be similar to the discussion on Infiltration and Inflow, to read about that Click Here
