I'm not quite sure which overflow design would work efficiently and at the same time be silent, I am mainly using the Herbie design but instead of using a stand pipe I'm drilling the holes on the back of the tank. Water should flow over my built in overflow box through the back of the tank and down to my sump. I'm going to use a single primary drain and will have an emergency drain just in case the primary gets clogged up for any reason. This overflow is for a 90 gallon setup so I'm looking for a 350-400 GPH flow rate.
I've take a few pics of the possible setups and would like some info on them. These are all gravitational overflows and there isn't any siphoning involved, there is no end cap or air hole either.
1)
Water flows down the primary without any air in the column and without any noise, should it malfunction the emergency would kick in causing a lot of noise to alert that there is something wrong with the primary.
2)
Without any extra fitting the water would flow through the bulkhead and down to the sump, a gate valve would be fixed to alter the flow if needed so the water level would be above the bulkhead and prevent any air from entering thus causing noise. The elbow fitting facing upwards on the emergency drain would grant a higher water level which would increase the water velocity on the primary and prevent air entrance.
3)
Providing the gravitational flow is enough, this should be a pretty easy design where water flows silently. No end cap or air hole is involved on the drain pump.
4)
This uses the same concept as option number 2 but without any fittings. I would use a gate valve on the primary drain(left) to adjust the water level between both drains.
Your input is appreciated.