Flow Equalization Tanks

1. Flow Equalization Tanks Presented by Keith Surface

2. Why use a Flow Equalization tank? • Peak Loading is a period when a disproportionate volume of wastewater is introduced into the ATU in relation to the overall expected flow. • Surges are incidents where wastewater is introduced into the ATU at a velocity that exceeds what the ATU can handle. • Peak Loading events and Surges are major causes of ATU disruption. • Peak loading events and Surges can: • 1. Reduce wastewater retention time in the ATU. • 2. “Wash out” the ATU. • 3. Raise BOD and TSS levels in the clarifier. • 4. Cause rapid sludge build up in the clarifier and pump tank. • 5. Create filter clogging issues in drip dispersal systems.

3. Advantages of Flow Equalization • A properly set up flow equalization tank can increase the ATU’s efficiency beyond the level it preformed during NSF testing. • NSF testing introduces wastewater at the following levels. • A. 25% over a three hour period • B. 35% over a three hour period • C. 40% over a three hour period • A properly set up flow equalization tank can equalize the daily flow over the entire twenty-four hour period… or week.

4. Can a Flow Equalization Tank reduce the size of the ATU that is required? • Chapter 285.32 Tank Sizing • (1) Tank sizing. Proprietary treatment systems that serve single family residences, combined flows from single family residences, or multi-unit residential developments shall be designed using Table II in §285.91(2) of this title unless there is an equalization tank preceding the aerobic treatment unit. If there is an equalization tank preceding the aerobic treatment unit, the equalization tank shall meet the requirements set forth in §285.34(b)(4) of this title (relating to Other Requirements) and the aerobic treatment units can be sized using the wastewater flows in Table III in §285.91(3) of this title. Harris County’s requirement of 150gpd of ATU treatment for each bedroom does not change when flow equalization is utilized.

5. Lowering ATU Size Requirements by Utilizing Flow Equalization • Example 1 • Chapter 285 Table II in §285.91(2) Without flow equalization: • A four bedroom residence less than 2501SF with a two bedroom guest house less than 1500SF - ATU treatment required would be 840GPD. • Chapter 285 Table III in §285.91(3) With flow equalization: • A four bedroom residence less than 2500SF with a two bedroom guest house less than 1500SF - ATU treatment required would be 480GPD • Example 2 • Chapter 285 Table II in §285.91(2) Without flow equalization: • A four bedroom residence just under than 5501SF with a two bedroom guest house less than 1500SF - ATU treatment required would be 1080GPD. • Chapter 285 Table III in §285.91(3) With flow equalization: • A four bedroom residence just under 5500SF with a two bedroom guest house less than 1500SF - ATU treatment required would be 600GPD

6. Lowering ATU Size Requirements by Utilizing Flow Equalization • Example 3 • Chapter 285 Table II in §285.91(2) Without flow-equalization: • A seven bedroom residence less than 7001SF • ATU treatment required would be 840GPD. • Chapter 285 Table III in §285.91(3) With flow-equalization: • A seven bedroom residence less than 7000SF • ATU treatment required would be 480GPD

7. Properly Setting Up a Flow Equalization Tank • Tank • Pump • Clock • Mechanism to control rate of flow

8. Tanks Chapter 285.34. Other Requirements The equalization tank must be preceded by a pretreatment tank; The tank shall be provided with an audible and visible high water alarm. The tank may contain a gravity line located above the high water alarm level which allows flow to the aerobic treatment unit.

9. Residential Flow Equalization Tank Sizing • Chapter 285.34. Other Requirements • (B) If an equalization tank is serving residences, the tank shall have • a volume between the pump intake level and the high water level of at least 50% of the design flow and be designed to time dose at equal intervals and equal doses throughout a 24-hour period. • Example: • Q=450GPD • 50% of Design Flow 450gpd X 50% = 225gal • Reserve above alarm 450gpd X 1/3 = 150gal • Tank Static Volume below Pump Off = 200gal • Flow-equalization tank size = 575gallons

10. Other Flow-Equalization Tank Sizing • Chapter 285.34 Other Requirements • (C) If an equalization tank is designed to equalize flows over periods • longer than a 24-hour period, the tank shall be designed to time dose at equal intervals and equal doses over the flow equalization time period. The design shall have a storage between the highest wastewater flow line during the period and the high level alarm equal to at least 20% of the flow generated during peak days. • Example: • Church Sunday Flow 2200gal • Highest Wastewater Flow Line = 2200gal • 2200gal X 20% = 440gal • Reserve above alarm 2200gal X 1/3 = 734gal (or 1/3 of average daily Q ?) • Operating volume required = 3374gal

11. PUMPS • Effluent Pump Flow Rates • 1/3hp effluent pump 44 - 55 gpm • 1/2hp effluent pump 90 - 120 gpm • 1hp effluent pump 150 - 180 gpm Chapter 285.34. Other Requirements (3) Pump specifications. A single pump may be used for flows equal to or less than 1,000 gallons per day. Dual pumps are required for flows greater than 1,000 gallons per day. An effluent filter in the pretreatment tank will solve many effluent pump issues. Sewage or solids pumps have less clogging issues but can create problems if flow is restricted.

12. Clocks Chapter 285.34. Other Requirements The design will use no fewer than 12 doses throughout the 24-hour period. (C) If an equalization tank is designed to equalize flows over periods longer than a 24-hour period, the tank shall be designed to time dose at equal intervals and equal doses over the flow equalization time period.

13. Rate of Flow Control Mechanism Consider the impact on an ATU caused by dumping a 55 gallon drum of wastewater directly into the aeration chamber. That is comparable to what an unrestricted effluent pump can do in 1- 2 minutes. • Mechanisms to adjust and monitor rate-of-flow. • 1. Ball valve followed by union in pump tank. • 2. Ball valve followed by union in ATU riser • 3. Water-meter with gate valve in valve box. • (standard meter tends to clog) • If installer or maintenance provider cannot demonstrate how they measure the rate-of-flow, they do not know what the actual rate-of-flow is!

14. Flow Equalization Rate-of-Flow NSF Standard 40 Testing – 5 gallons of residential strength wastewater are introduced into 500GPD ATU through a dump station and piping mechanism 100 times a day. Morning: 35 dumps - 1 dump approx. every 5.1 minutes for 3 hrs (average = 0.98gpm) Afternoon: 25 dumps - 1 dump approx. every 7.2 minutes for 3 hrs (average = 0.69gpm) Evening: 40 dumps - 1 dump approx. every 4.5 minutes for 3 hrs (average = 1.1gpm) Most Manufacturer’s Recommended dosing flow for a 500GPD ATU is 1- 5 GPM. Effluent Pump Flow Rates 1/3hp effluent pump - 44-55gpm 1/2hp effluent pump - 90-120gpm 1hp effluent pump - 150-180gpm Lift Station: A lift Station that has 12inches between the on and off switch can discharge 200gallons in 4 minutes. (Based on 16.5 gallons per inch)

15. Flow-equalization for multiple ATU’s Adjustment and measurement