inlet location to tank 1

[drain]

Hi,
1) We're designing a 1250m3, 2-cell un-baffled rectangular ground level concrete tank, 3.6m is the TWL. We're debating the issues of inlet location with respect to water turnover and chlorine residual. We've currently got our inlet discharging through an elbow downward at TWL. Any comments?

2) This issue concerns the tank's overflow discharge. Ten State Standards wants 1 -2 feet above grade for visibility. We've got Dept of Fisheries to satisfy that we're not putting chlorinated water into the creeks. Similar to item 1), but now we want vaporization, would a splash pad cause enough vaporization of chlorine to occur?

 

[waterexpert]

My first comment would be that a baffled tank will give far better control of residence times, and hence disinfection. Here in the UK we have an old standard for the design of these tanks, WrC TR60.

 

[drain]

Thanks for the comment. I've searced the WRc site but can't find any reference to TR60. Is it something voluminous or could I request they email or fax me?
I'm interested also to know where the doc indicates discharge elevation to be, at the bottom, midway or at the top.

 

[waterexpert]

Not too voluminous, but I don't think it's still in print.

Full reference is "Technical Report TR60: Disinfection by chlorination in contact tanks by J.G Naughton and R.Gregory December 1977"

The most important bits follow:

Firstly, introduction of water into the chlorine contact tank:

"the major problem at this stage of treatd water flow is the need to produce a more uniform flow across the channel in as short a distance as possible. This is sualully acheive dby rapid reduction in the excess energy of the incoming floew to small scale turbulence.

An example of poor inlet design is that of a pipe aligned along the longitudinal axis of the first bay of a contact tank. With this arrangement, retention time in the bay can be reduced to one twentieth of the theoretical residence time..Studies..favour an inlet design incorporating a baffle to break up the jetand a further perforated baffle to smooth the flow..one way of acheiving this baffle effect is to introduce the inlet pipe at right angles into the bay.
..studies..favour narrow bays..give better performance in flow distribution."

More generally,they go on to recommend a rectangular channel with width to length ratio of 40:1. This is acheived in a compact space by forming four or more 10:1 ratio channels joined by 180 degree bends. A minimum theoretical hydraulic residence time of 30 minutes is recommended, at a free chlorine dose of 1-2mg/l.

 

[waterexpert]

Oh, and the above assumes pH 6-8 is maintained.

 

[drain]

Our water is chlorinated off site and the issue is whether I discharge into the tank at the TWL or somewhere below it. For circulation, the riser pipe has an elbow directed down and to an angle to promote a swirling action. Ideally, we'd have a circular tank with no dead spots. I appreciate your inputs. Thank you.

 

[waterexpert]

OK-my mistake,if I now understand your question correctly, this is just a storage tank, not a chlorine contact tank.

You will not vaporise chlorine with a baffle. You will need to get a dechlorination chamber. These hold tablets which liberate chlorine destroying chemical into any water flowing through. I've used quite a few of these for river discharge of chlorinated stored water in the UK.

 

[drain]

We want to keep the chlorine residual and I know that it changes state during vaporization, which could occur during discharge, but I don't know how much. During a shower, when the water is hot, change of state occurs readily, but perhaps I should also consult a phase diagram for chlorine and see how much temperature dependent it is.

I also want to promote circulation to minimize dead spots and so am concerned about the location of the tank inlet pipe. Both these concerns affect where the discharge is located: at the top of the tank or somewhere below.

The second issue in my original posting related to dechlorination and also to making the condition of tank overflow physically visible. In the overflow condition, we want dechlorination to occur.

Sorry if it the issues weren't clearly described.

 

[BobPE]

what are you doing with this tank? Is is a distribution system tank? a process tank? for water? for industry? Does the distribution system float off the tank gradient?

What makes you think you can vaporize the chlorine. The reaction to get chlorine out of water by vaporization is very long and therefore not very usefull in normal water engineering applications.

The inlet to the pipe at the top would not be my first choice, but that is based on it being a water storage tank.

BobPE

 

[drain]

Bob,

Tank is potable water storage for distribution for a residential development.

 

[BobPE]

why are ther two cells? is pressure to the residential community being set by the tank?

BobPE

 

[drain]

yes. Water utility requires 2 cells so one can be taken out for service. Gravity flow to consumers, controlled by water levels.

 

[BobPE]

I wouldn't recommend ballfing, thats about the worst thing you can do since it may decrease mixing time and will maxamize residence times. Is there a common influent/effluent pipe, i am assuming not. I would look to mix the tank contents with the influent pipe, making placing it at the bottom of the tank an ideal design. Look to minimize short circuiting, but not using baffles. Space the effluent from the influent.

I dont follow ten states, they are good for wastewater, but I think they are a joke when it comes to water, but that is personal preference.

There was another post here talking about a declorination chamber, that is a good design you will have to put in. The last one I did used a chem feed system with sodium bisulfite into a stilling pool. You will have to consider this seriously since the fish commission carry guns and can place the operator in jail.

BobPE

 

[waterexpert]

Dechlorination chambers carry a stack of tablets at small scale, or use liquid chemicals at larger scale. You can get a prefabricated one for small scale, (which it sounds like you have)from a fish pond supplies outfit.

 

[BobPE]

water expert: good ideas, water systems usually have overflow potentials of hundreds of gallons a minute, even for small communities, making the fish store out of the question. The problem with tablets is they may not accomodate larger flows without being tended by someone. They are great for hydrant flushing where someone is there to refil the dispensers. You are right though, something must be done with the chlorine. My most recent design was for a smaller 100,000 gallon tank, I had to put in a liquid system, in a building designed for only that system, a RTU to control it, a liquid sensor to sense the overflow occuring, chemical feed pumps, heaters, lights, etc. It was a complete waste of money, but was all required by the state DEP. I did the overland flow calcs from the overflow to the receiving stream and showed that the chlorine demand was suficient enough to take out all the chlorine before the water reached the stream, even for long periods of time at high foverflows. No avail, the state wanted the system anyway.

BobPE

BobPE

 

[waterexpert]

I suppose it depends where you are,and what the regulatory authorities are willing to put up with. I've used the tablet dispensers for overflows from the main storage from 3.5 MLD water treatment plants. They come in quite large sizes( up to maybe 6" nominal bore). These large units are not however likely to be stocked by pool shops.

You are right in saying they aren't much use if no-one fills the chamber.You can have a simple mechanical system to warn that an overflow has taken place. I am assuming that overflows from this tank are not common occurrences.

I'd need to know how large the maximum overflow rate was to decide between a no moving parts system and the full chemical dosing system you describe. It'll certainly do the job, but as you say, it's a waste of money.

 

[drain]

overflow rate could reach 2000 USgpm

 

[waterexpert]

So you probably have about a 12" overflow pipe? I'd go with the liquid system, as a minimum comprising chlorine residual monitoring controlling a single dosing pump and IBC of sodium metabisulphite. You could use a static mixer to ensure the dose is evenly mixed in, but this would probably require another 3m of head over and above that to overcome line losses, and your tank ends up pretty high in the air.

As BobPE mentions in passing, if you can flow over a decent run of grassland before the watercourse, this will deactivate the chlorine.

Alternatively, you could consider a granular activated carbon bed.

 

[NCTANK]

I would suggest a circular prestressed concrete tank and separating the inlet from the outlet 180 degrees. The inlet pipe should rise above the floor to a level below the normal operating range. The outlet pipe should be just above the floor. If additional mixing is required there are various types of diffuser piping systems that could be incorporated.

 

[BobPE]

NCTANK:

DOesn't putting the influent pipe towards the top waste the velocity head energy of the fluid entering the tank and reduce mixing? I've modeled the influent and opposite effluent for several tanks and it isn't an efficient mix design when you look at it on paper.

Just some thoughts...

BobPE