Interconnecting treated water system

[IMNO]

Hello every one,
I am currently involved in a project which has several Sewage treatment plants (STP) to treat sewage and use it for irrigation of landscape, parks; median etc. The project area is around 15000 acre. There are total 10 STPs. Some are located closer to each other while some are at longer distance to each other. The project is divided into different blocks. The treated water from STPs will be pumped into distribution tanks, proposed in each blocks, which eventually be used for irrigating that particular block. The client wants to interconnect the STPs so that in case of low sewage generation or emergency, the treated water from any STP can be used to fulfil the irrigation need of other area.
I am planning to provide Wet well and pump station at each STP so that treated water from STP will enter the wet well through gravity.
Option 1:
One possibility is to connect all wet well together in close loop so that each can transfer treated water to each other. Further each wet well will be responsible to fill certain tanks closer to that wet well through branch network.
For example there are 10 wet well are all connected through loop network.
Wet well 1 can supply treated water to distribution tank 1,2,3,4 while wet well 2 can supply treated water to tank 5,6,7,8.
Incase if wet well 1 do not have sufficient treated water than it cannot supply to distribution tank 1,2,3,4 until it get water from some other wet well.

Option 2:
All the wet wells and distribution tanks will be connected into a closed loop pipe network.
Each wet well will have float switches, as the water level rises to certain depth the pumps will start pumping into a closed loop network and distribution tanks will start filling. Each distribution tank will have modulating float valve to avoid overflow of tank and maintain pressure in the line. As the tanks get filled the pressure in the system will rise which eventually stop the pump.

All pumps will be VFD based
Has anyone designed such scheme before?
What are pros and cons of each option?
What should be the best way to operate such scheme?
I want to discuss the view of other regarding the scheme and what would be the best scheme in such scenario?

Thanks

 

[bimr]

What is the volume of wastewater, population, and climate?

The initial thought is that there are too many treatment plants which adds to the cost and complexity to operate.

 

[IMNO]

The approximate waste water would be 22 MGD, while the climate is moderate

 

[ashtree]

For that flow and population it would be far better to have one treatment plant. Most likely in terms of both short and long term cost it would be cheaper despite the need to have trunk mains that take collected sewerage from smaller pump stations to the treatment plant.

However it is my guess that this will be some sort of staged development and the developer only wants to put in the infrastructure for that stage and intends to build a small collection system and treatment plant for that section rather than to install the trunk infrastructure to suit the ultimate development. Whilst the developers efforts to recycle the water should be applauded its probably also an attempt to do things at a lower cost by not having as rigid permitting conditions.

It is most likely this will a long term technical and financial nightmare. I have seen similar schemes where this sort of development is set up as a stand alone "Sewer and Recycled Water Utility" which holds all the debt for the development of the scheme as well as the responsibility for Operation and Maintenance. Last time i looked 19 of the 20 schemes i knew about had gone bankrupt and the local authority had to step in.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"