McDonald Group International, Inc

Water, Wastewater, Environmental Engineering 

 

 

Problem Description

Often times poor treatment from a sewage treatment plant is blamed on hydraulic surges.

During the course of a day, the amount of sewage reaching the facility will vary. In a predominantly residential area there typically there is little flow at night, a heavy peak flow during the morning, followed by a period in which it subsides, followed by another peak later in the day.

The major problems heavy peak flows can cause a treatment plant are as follows:

1. Billowing Sludge: Sludge settling in the settling tank or clarifier is billowed up by an excessive amount of water entering the tank, which can rise to the surface and wash over into the effluent launder and out of the plant.

2. Weakening of the BioMass: Most activated sludge wastewater plants maintain between 2500 and 4000 mg/L suspended solids in the aeration tank. An excessive amount of raw sewage can cause the concentration to thin out, which destabilizes the biological process


Investigate the Problem

First, make sure you have a surge problem and not something else.

If the sludge in the plant has poor settling characteristics, then even relatively low peak flows are apt to cause settling problems. Run a settling test early in the morning, after the plant has had a long period of low flow and is reasonably stable, before the morning peak hits. If the sludge settles poorly even then, then dampening peak or surge flows is not likely to be of much help.

Next, try to determine what the current peak rate of flow actually is.

Few small or medium sized wastewater plants have diurnal chart recorders. Those that do have them usually record only the effluent flowrate, which often times records only the flow of water out of the plant when the plant aerators shut off. (The air supplied lowers the density of the water in the aeration tanks, causing the liquid to rise several inches. When the blowers shut off, the liquid column collapses, sending a mini surge of water out of the plant).

However, it is often possible to check the master water supply meter and to take hourly readings of it. Make note of what the largest hourly flow was , and divide it by the total flow for the day to determine what the ratio of peak to average flow is.

Another method we have used for wastewater plants that have sewage flows pumped to them is to attach an event chart recorder to the control panel (call us if interested in doing this) of the pumps which records when and how long the pumps run. From this, we can construct the pattern of flow through the facility.

In our experience, the ratio of peak to average flow at various kinds of facilities has been:

1) Shopping Centers 2:1 (the peak flow was surprisingly but is sustained for several hours)

2) Schools 5:1 (the peak occurs during the lunch hour)

3) Residential facilities between 2:1 and 3:1.

There are a number of publications that provide suggested peak flows. However, the decision to provide surge protection on a wastewater plant should be based on good solid data as to the true magnitude of the surge.

Next measure the surface area of the settling tank. Multiple the peak flow ratio times the average flow for the day and divide by the measured surface area of the settling tank(s). If the number is not over 750 gpd/sf, it is unlikely that the plant is suffering from a surge flow.

Exception: If the period of peak flow lasts for more than two hours, then it is possible that the plant can suffer problems form hydraulic surge, even if the peak flow to average flow ratio is small.

One last thing needs to be checked. If the pumps in the lift station are oversized, and there is a long elapsed time between pump cycles, then the pumps may be surging the plant when they come on. Throttling the discharge and providing for more frequent dosing of the plant may help eliminate the problem.


 

Solution

There are three solutions to hydraulic surge problems:

1) Reduce the influent flow from upstream lift stations (if possible)

2) Make the sewage plant big enough to handle the periods of heavy flow (too expensive)

3) Add a surge tank

A common rule of thumb is that the surge tank should be equal in volume to 15% of the total flow coming to the plant. Depending on the actual flow characteristics of the institution and how the surge tank is designed, that may or may not work.

Surge tanks should be designed to dampen the influent rate of flow to about 150% of average daily flow or whatever smaller number is needed to assure that the sewage plant will not be overloaded based on an analysis of the size of the components.

Diurnal flow curve in a facility with influent peak flows three times ADF, and showing the volume in the surge tank used to dampen the flows to 1.5 times ADF

Diurnal flow curve in a facility with influent peak flows twice ADF, and showing the volume in the surge tank used to dampen the flows to 1.5 times ADF

The actual volume is determined on a mass balance calculation Based on a typical distribution of flow during the day, for reducing a peak to average flow ratio from 2:1 to 1.5:1, the required volume in surge tank is about 10% of the flow. For damping a peak ratio of 3:1 to 1.5:1, the required surge volume is about 18% of the flow.


 

Surge Tank Design

Surge tanks can either be designed "in line" or "offline".

In the former case, all the influent is directed to the surge tank. Pumps inside the tank then transfer flow out of the surge tank to the treatment plant.

In an offline configuration, the flow is first directed to a flow splitter box in which there are weirs of different heights and sizes. A portion of all the flow that comes into the box always goes to the treatment plant. Only surge flows of a certain magnitude go into the surge tank. The tank is later pumped down during periods of low flow.

It is not unusual to see surge tanks, pumps, and splitter boxes either not designed properly or not set properly.

A notorious example we encountered representative of both errors was a 30,000 gallon in line surge tank that was provided for a 50,000 gallon a day plant. Pumps in the surge tank were oversized and set to pump into a splitter box, which was intended to return to the surge tank a certain amount of the surge pump flow and was supposed to send to the plant a reduced flow. However, the weirs in the splitter box were set so that all of the surge pump flow was sent to the plant. It was as if there were no surge tank on the plant at all.

Finally, here is an example of how to set the weirs in that particular plant:

Plant Flow, ADF 50,000 gpd
Surge Pump Flow Rate: 75 gpm
Length of Return Flow Weir 8 inches
Length of Plant Flow Weir 6 inches
Plant Flow, as gpm 34.72 gpm
150% of average flow  52.08 gpm
150% of average flow, as CFS 0.12 cfs
Return Flow Rate 22.92 gpm
Return Flow Rate 0.05 cfs
Depth of Flow over  Plant Weir   2.09 inches
Depth of Flow over Return Weir 1.00 inches

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