Dosing tanks-Keith Surface, Surface Septic Systems

Report
Flow Equalization Tanks
Presented by
Keith Surface
Why use a Flow Equalization tank?
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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:
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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.
Advantages of Flow Equalization
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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.
Can a Flow Equalization Tank reduce
the size of the ATU that is required?
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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.
Lowering ATU Size Requirements by
Utilizing Flow Equalization
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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.
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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
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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.
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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
Lowering ATU Size Requirements by
Utilizing Flow Equalization
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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
Properly Setting Up a Flow
Equalization Tank
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Tank
Pump
Clock
Mechanism to control rate of flow
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.
Residential Flow Equalization Tank
Sizing
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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
Other Flow-Equalization Tank Sizing
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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.
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Example:
Church Sunday Flow 2200gal
Highest Wastewater Flow Line
2200gal X 20%
Reserve above alarm 2200gal X 1/3
Operating volume required
= 2200gal
= 440gal
= 734gal (or 1/3 of average daily Q ?)
= 3374gal
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.
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.
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!
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)
Flow-equalization for multiple ATU’s
Adjustment and measurement

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