Tables: Cisterns (Small-Scale GI)
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1. Table 1
BMP 2021, Ch 9.1, p. 1
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards |
|
|
|
Green Infrastructure |
Yes |
|
Stormwater Runoff Quantity |
Yes |
|
Groundwater Recharge |
Not Allowed |
|
Stormwater Runoff Quality |
Not Allowed |
Source context and notes
Source revision: March 2021.
# 9.1 CISTERNS
[Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1)
Cisterns are stormwater facilities that temporarily store stormwater runoff from clean rooftops, which is subsequently reused for non-potable uses, such as toilet flushing and vehicle washing. Cisterns can either be indoors or outdoors and above, at or even below grade. The reuse of collected stormwater runoff reduces the volume of stormwater runoff transported to downstream facilities. Take note that cisterns are not a viable stormwater management choice for sites with little or no demand for reuse. Cisterns are assigned a 0% total suspended solids (TSS) removal rate.
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | |
| --- | --- | --- |
| | Green Infrastructure | Yes |
| | Stormwater Runoff Quantity | Yes |
| | Groundwater Recharge | Not Allowed |
| | Stormwater Runoff Quality | Not Allowed |
# Introduction
Cisterns are stormwater management practices used to capture, collect and reuse stormwater runoff from clean roofs. Cisterns are ideal for harvesting rainwater for non-potable uses, vehicle washing or toilet flushing. Cisterns are extremely versatile and may be used on a variety of sites ranging from small-scale residential sites to large-scale industrial or commercial sites; they may be placed either indoors or outdoors and above, at or below grade. They can also be used in various shapes and sizes. Cisterns must be designed based upon on-site water needs; an under-sized cistern may not store sufficient water for site demands. An over-sized cistern may remain full or near-full most of the time, and thus, be unable to provide storage during rain events.
A cistern must have a maintenance plan and must be reflected in a deed notice recorded in the county clerk's office to prevent alteration or removal.
2. Table 2
BMP 2021, Ch 9.1, p. 10
| Roof Area |
2,500 sf |
| First Flush Diversion |
0.01 inches |
| Weekday Building Occupancy |
100 |
| Weekend Building Occupancy |
50 |
Source context and notes
A cistern stores runoff generated by the WQDS to be reused over the course of three (3) days, following stormwater capture. Therefore, it is necessary to ensure that the cistern stormwater reuse demand at the project site is sufficient to ensure that the cistern is emptied in time for the next rain event. A cistern-analysis Excel spreadsheet intended to help size a cistern for site-specific conditions may be found below the subheading for Chapter 9.1 on the Department's webpage at:
https://njstormwater.org/bmp_manual2.htm.
The following examples illustrate how to use a cistern to reduce the volume of runoff generated by a rooftop and how to use the spreadsheet to aid in the sizing of the cistern. These examples show possible configurations and flow paths and are not intended to limit the design.
Example 1: The runoff from a 2,500 sf roof will be collected in a subsurface cistern and will be reused inside a building to flush toilets and urinals. The following parameters apply:
| Roof Area | 2,500 sf |
| --- | --- |
| First Flush Diversion | 0.01 inches |
| Weekday Building Occupancy | 100 |
| Weekend Building Occupancy | 50 |
3. Table 3
BMP 2021, Ch 9.1, p. 11
| A |
|
| 1 |
INPUTS |
| 2 |
|
| 3 |
WHAT IS THE ROOF AREA THAT DRAINS TO THE CISTERN? (SQUARE FEET) |
| 4 |
2,500.00 |
| 5 |
WHAT AMOUNT OF RUNOFF IS DIVERTED BY THE FIRST FLUSH DIVERTER? (INCHES) |
| 6 |
0.01 |
| 7 |
CURVE NUMBER ROOF - ASSUMED 98, UNLESS USING A GREEN ROOF |
| 8 |
98 |
Source context and notes
Source revision: March 2021.
Step 1: Input Basic Parameters
Begin on the first worksheet of the cistern sizing spreadsheet, entitled INPUTS AND RESULTS. The first input is the area of the roof that will drain to the cistern; for this example, the area of the rooftop is 2,500 sf. The next is the amount of rainfall that will be diverted by the first flush diverter; which is set at at 0.01 inches. The final input that is needed is the curve number of the roof, which in most situations will be 98, unless a green roof is being used.
Initial Inputs
| A | |
| --- | --- |
| 1 | INPUTS |
| 2 | |
| 3 | WHAT IS THE ROOF AREA THAT DRAINS TO THE CISTERN? (SQUARE FEET) |
| 4 | 2,500.00 |
| 5 | WHAT AMOUNT OF RUNOFF IS DIVERTED BY THE FIRST FLUSH DIVERTER? (INCHES) |
| 6 | 0.01 |
| 7 | CURVE NUMBER ROOF - ASSUMED 98, UNLESS USING A GREEN ROOF |
| 8 | 98 |
Step 2: Input Stormwater Reuse Information
Next, move on to the second worksheet, entitled WATER USE (TOILETS). The first set of inputs will determine the daily toilet/urinal use based on 1.6 gallons/toilet flush and 1.0 gallons/urinal flush; these inputs include the number of people that use the building, the percentage of users that exclusively use toilets and the percentage of weekday use that is expected on weekends. This worksheet assumes 3 flushes/8-hour workday/person. It also assumes that of those 3 flushes by males 2 are for urinals and 1 is for toilets. If the water use of toilets and urinals in the building are known, and differ from these default usage numbers, these can also be directly input into the worksheet.
Toilet/Urinal Reuse
4. Table 4
BMP 2021, Ch 9.1, p. 11
|
A |
B |
C |
| 1 |
CALCULATED TOILET USE PER DAY (GALLONS) |
|
|
| 2 |
|
|
HOW MANY PEOPLE USE THE BUILDING? |
| 3 |
WEEKDAY |
|
100 |
| 4 |
840.00 |
|
WHAT PERCENTAGE OF THE PEOPLE EXCLUSIVELY USE TOILETS? |
| 5 |
WEEKEND |
|
50 |
| 6 |
420.00 |
|
HOW MUCH WATER IS USED BY EACH TOILET? (GALLONS/FLUSH) |
| 7 |
|
|
1.6 |
| 8 |
|
|
HOW MUCH WATER IS USED BY EACH URINAL? (GALLONS/FLUSH) |
| 9 |
|
|
1 |
| 10 |
|
|
HOW MANY HOURS PER DAY IS THE BUILDING USED? |
| 11 |
|
|
16 |
| 12 |
|
|
WHAT PERCENTAGE OF WEEKEDAY USE IS EXPECTED ON WEEKENDS? |
| 13 |
|
|
50 |
Source context and notes
| 8 | 98 |
Step 2: Input Stormwater Reuse Information
Next, move on to the second worksheet, entitled WATER USE (TOILETS). The first set of inputs will determine the daily toilet/urinal use based on 1.6 gallons/toilet flush and 1.0 gallons/urinal flush; these inputs include the number of people that use the building, the percentage of users that exclusively use toilets and the percentage of weekday use that is expected on weekends. This worksheet assumes 3 flushes/8-hour workday/person. It also assumes that of those 3 flushes by males 2 are for urinals and 1 is for toilets. If the water use of toilets and urinals in the building are known, and differ from these default usage numbers, these can also be directly input into the worksheet.
Toilet/Urinal Reuse
| | A | B | C |
| --- | --- | --- | --- |
| 1 | CALCULATED TOILET USE PER DAY (GALLONS) | | |
| 2 | | | HOW MANY PEOPLE USE THE BUILDING? |
| 3 | WEEKDAY | | 100 |
| 4 | 840.00 | | WHAT PERCENTAGE OF THE PEOPLE EXCLUSIVELY USE TOILETS? |
| 5 | WEEKEND | | 50 |
| 6 | 420.00 | | HOW MUCH WATER IS USED BY EACH TOILET? (GALLONS/FLUSH) |
| 7 | | | 1.6 |
| 8 | | | HOW MUCH WATER IS USED BY EACH URINAL? (GALLONS/FLUSH) |
| 9 | | | 1 |
| 10 | | | HOW MANY HOURS PER DAY IS THE BUILDING USED? |
| 11 | | | 16 |
| 12 | | | WHAT PERCENTAGE OF WEEKEDAY USE IS EXPECTED ON WEEKENDS? |
| 13 | | | 50 |
5. Table 5
BMP 2021, Ch 9.1, p. 12
| 9 |
MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (GALLONS) |
|
| 10 |
1,680.00 |
|
| 11 |
MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (CUBIC FEET) |
|
| 12 |
224.58 |
|
| 13 |
WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (GALLONS) |
|
| 14 |
1,596.74 |
|
| 15 |
WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (CUBIC FEET) |
|
| 16 |
213.45 |
|
| 17 |
|
|
| 18 |
RESULT - 1ST STEP |
|
| 19 |
BUILDING'S WATER USAGE IS SUFFICIENT TO JUSTIFY A CISTERN |
|
| 20 |
1,294.41 |
GALLONS |
Source context and notes
Source revision: March 2021.
Step 3: Review of Cistern Size
Once all the information is entered, proceed back to the first worksheet, entitled INPUTS AND RESULTS. The green cells on the left hand side will show both the minimum volume of water the building will use in a three (3) day period and the minimum capacity of the cistern based on the WQDS in both gallons and cubic feet. The spreadsheet will identify if the building's water usage is sufficient to justify the size of the cistern. It is important to review the calculated information to determine if the parameters are appropriate.
Results
| 9 | MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (GALLONS) | |
| --- | --- | --- |
| 10 | 1,680.00 | |
| 11 | MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (CUBIC FEET) | |
| 12 | 224.58 | |
| 13 | WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (GALLONS) | |
| 14 | 1,596.74 | |
| 15 | WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (CUBIC FEET) | |
| 16 | 213.45 | |
| 17 | | |
| 18 | RESULT - 1ST STEP | |
| 19 | BUILDING'S WATER USAGE IS SUFFICIENT TO JUSTIFY A CISTERN | |
| 20 | 1,294.41 | GALLONS |
Step 4: Optional Detention for Larger Storm Events
If desired, the cistern may also be used for the detention of larger storm events. The volume calculated by the spreadsheet would be located below the quantity control outlet(s) of the cistern and a temporary storage volume above the outlet(s) would be used to provide detention. This temporary storage volume would be sized using a routing calculation. For additional information on the design, operation and maintenance of the extended detention components refer to Chapter 11.2: Extended Detention Basins.
Example 2: The runoff from a 3,000 sf roof will be collected in a subsurface cistern and will be reused inside the building to flush toilets and urinals and can also be used for vehicle washing. The following parameters apply:
6. Table 6
BMP 2021, Ch 9.1, p. 13
|
A |
| 1 |
INPUTS |
| 2 |
|
| 3 |
WHAT IS THE ROOF AREA THAT DRAINS TO THE CISTERN? (SQUARE FEET) |
| 4 |
3,000.00 |
| 5 |
WHAT AMOUNT OF RUNOFF IS DIVERTED BY THE FIRST FLUSH DIVERTER? (INCHES) |
| 6 |
0.01 |
| 7 |
CURVE NUMBER ROOF - ASSUMED 98, UNLESS USING A GREEN ROOF |
| 8 |
98 |
Source context and notes
# Cisterns (Small-Scale GI)
Source revision: March 2021.
final input that is needed is the curve number of the roof, which as mentioned previously in most situations will be 98, unless a green roof is being used.
## Initial Inputs
| | A |
| --- | --- |
| 1 | INPUTS |
| 2 | |
| 3 | WHAT IS THE ROOF AREA THAT DRAINS TO THE CISTERN? (SQUARE FEET) |
| 4 | 3,000.00 |
| 5 | WHAT AMOUNT OF RUNOFF IS DIVERTED BY THE FIRST FLUSH DIVERTER? (INCHES) |
| 6 | 0.01 |
| 7 | CURVE NUMBER ROOF - ASSUMED 98, UNLESS USING A GREEN ROOF |
| 8 | 98 |
## Step 2: Input Water Reuse Information
As done in the previous example, enter the appropriate information in the orange cells on the second worksheet, entitled WATER USE (TOILETS). These numbers correspond with the numbers used in the previous example. Remember, if the water use of toilets and urinals in the building is known and differs from the default usage numbers, these values can also be directly input into the worksheet.
### Toilet/Urinal Reuse
7. Table 7
BMP 2021, Ch 9.1, p. 13
|
A |
B |
C |
| 1 |
CALCULATED TOILET USE PER DAY (GALLONS) |
|
|
| 2 |
|
|
HOW MANY PEOPLE USE THE BUILDING? |
| 3 |
WEEKDAY |
|
100 |
| 4 |
840.00 |
|
WHAT PERCENTAGE OF THE PEOPLE EXCLUSIVELY USE TOILETS? |
| 5 |
WEEKEND |
|
50 |
| 6 |
420.00 |
|
HOW MUCH WATER IS USED BY EACH TOILET? (GALLONS/FLUSH) |
| 7 |
|
|
1.6 |
| 8 |
|
|
HOW MUCH WATER IS USED BY EACH URINAL? (GALLONS/FLUSH) |
| 9 |
|
|
1 |
| 10 |
|
|
HOW MANY HOURS PER DAY IS THE BUILDING USED? |
| 11 |
|
|
16 |
| 12 |
|
|
WHAT PERCENTAGE OF WEEKEDAY USE IS EXPECTED ON WEEKENDS? |
| 13 |
|
|
50 |
Source context and notes
| 8 | 98 |
## Step 2: Input Water Reuse Information
As done in the previous example, enter the appropriate information in the orange cells on the second worksheet, entitled WATER USE (TOILETS). These numbers correspond with the numbers used in the previous example. Remember, if the water use of toilets and urinals in the building is known and differs from the default usage numbers, these values can also be directly input into the worksheet.
### Toilet/Urinal Reuse
| | A | B | C |
| --- | --- | --- | --- |
| 1 | CALCULATED TOILET USE PER DAY (GALLONS) | | |
| 2 | | | HOW MANY PEOPLE USE THE BUILDING? |
| 3 | WEEKDAY | | 100 |
| 4 | 840.00 | | WHAT PERCENTAGE OF THE PEOPLE EXCLUSIVELY USE TOILETS? |
| 5 | WEEKEND | | 50 |
| 6 | 420.00 | | HOW MUCH WATER IS USED BY EACH TOILET? (GALLONS/FLUSH) |
| 7 | | | 1.6 |
| 8 | | | HOW MUCH WATER IS USED BY EACH URINAL? (GALLONS/FLUSH) |
| 9 | | | 1 |
| 10 | | | HOW MANY HOURS PER DAY IS THE BUILDING USED? |
| 11 | | | 16 |
| 12 | | | WHAT PERCENTAGE OF WEEKEDAY USE IS EXPECTED ON WEEKENDS? |
| 13 | | | 50 |
## Step 3: Review of Cistern Size
As done in the previous example, once all the information is entered, proceed back to the first worksheet, entitled INPUTS AND RESULTS. The green cells will show both the minimum volume of water the building will use in a three (3) day period and the minimum capacity of the cistern based on the WQDS in both gallons and cubic feet. The spreadsheet will identify if the building's water usage is sufficient to justify the size of the cistern. It is important to review the calculated information to determine if the parameters are appropriate. The results are shown on the following page.
8. Table 8
BMP 2021, Ch 9.1, p. 14
| 9 |
MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (GALLONS) |
| 10 |
1,680.00 |
| 11 |
MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (CUBIC FEET) |
| 12 |
224.58 |
| 13 |
WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (GALLONS) |
| 14 |
1,916.08 |
| 15 |
WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (CUBIC FEET) |
| 16 |
256.14 |
| 17 |
|
| 18 |
RESULT - 1ST STEP |
| 19 |
BUILDING'S WATER USAGE IS INSUFFICIENT TO JUSTIFY A CISTERN |
| 20 |
|
| 21 |
RESULT - 2ND STEP |
Source context and notes
# Cisterns (Small-Scale GI)
Source revision: March 2021.
# Results
| 9 | MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (GALLONS) |
| --- | --- |
| 10 | 1,680.00 |
| 11 | MINIMUM VOLUME OF WATER, BUILDING UTILIZES DURING A THREE (3) DAY PERIOD. (CUBIC FEET) |
| 12 | 224.58 |
| 13 | WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (GALLONS) |
| 14 | 1,916.08 |
| 15 | WATER QUALITY DESIGN STORM VOLUME (CISTERN'S MINIMUM CAPACITY) (CUBIC FEET) |
| 16 | 256.14 |
| 17 | |
| 18 | RESULT - 1ST STEP |
| 19 | BUILDING'S WATER USAGE IS INSUFFICIENT TO JUSTIFY A CISTERN |
| 20 | |
| 21 | RESULT - 2ND STEP |
# Step 4: Toilet/Urinal Reuse Not Sufficient
Unlike in the previous example, here the building's lowest 3-day cistern stormwater reuse (toilet & urinals) is not sufficient to empty the cistern. It is important to note that, since the building in this example has lower cistern stormwater reuse demand on weekends, those lower weekend numbers were used to determine if the cistern would be emptied after 3 days. If the average demand is high enough to empty the cistern in 3 days, then the use of a cistern is still possible, but will require a secondary storage tank.
# Step 5: Secondary Storage Tank
9. Table 9
BMP 2021, Ch 9.1, p. 15
| HOW MUCH WATER WILL BE USED TO WASH VEHICLES? (GALLONS) |
|
|
WEEKLY AMOUNT (GALLONS) |
| MONDAY |
|
|
1,000.00 |
|
0.00 |
|
|
| TUESDAY |
|
|
|
|
0.00 |
|
|
| WEDNESDAY |
|
|
|
|
0.00 |
|
|
| THURSDAY |
|
|
|
|
0.00 |
|
|
| FRIDAY |
|
|
|
|
1,000.00 |
|
|
| SATURDAY |
|
|
|
|
0.00 |
|
|
| SUNDAY |
|
|
|
|
0.00 |
|
|
Source context and notes
# Cisterns (Small-Scale GI)
Source revision: March 2021.
Vehicle Washing Reuse
| HOW MUCH WATER WILL BE USED TO WASH VEHICLES? (GALLONS) | | | WEEKLY AMOUNT (GALLONS) |
| --- | --- | --- | --- |
| MONDAY | | | 1,000.00 |
| | 0.00 | | |
| TUESDAY | | | |
| | 0.00 | | |
| WEDNESDAY | | | |
| | 0.00 | | |
| THURSDAY | | | |
| | 0.00 | | |
| FRIDAY | | | |
| | 1,000.00 | | |
| SATURDAY | | | |
| | 0.00 | | |
| SUNDAY | | | |
| | 0.00 | | |
## Step 7: Secondary Storage of Cistern Size
As done earlier in this example, once all the information is entered, proceed back to the first worksheet, entitled INPUTS AND RESULTS. There is now a new field called Result – 2nd Step. The spreadsheet will identify if the building’s additional water usage is sufficient to justify the size of the cistern. It is important to review the calculated information to determine if the parameters are appropriate. Note that numerical value appearing below the yellow cell identifies the minimum size of the temporary storage tank necessary to allow the cistern to work appropriately. Here, the additional water usage is sufficient to justify a cistern, as long as an additional storage tank that stores approximately 1,703 gallons is also constructed to ensure the cistern is able to be emptied within a week.
## Results
10. Table 10
BMP 2021, Ch 9.1, p. 15
| RESULT - 2ND STEP |
|
| ADDITIONAL WATER USAGE IS SUFFICIENT TO JUSTIFY A CISTERN |
|
| ADDITIONAL STORAGE TANK NECESSARY (GALLONS) |
|
| 1,703.08 GALLONS |
|
Source context and notes
| | 0.00 | | |
## Step 7: Secondary Storage of Cistern Size
As done earlier in this example, once all the information is entered, proceed back to the first worksheet, entitled INPUTS AND RESULTS. There is now a new field called Result – 2nd Step. The spreadsheet will identify if the building’s additional water usage is sufficient to justify the size of the cistern. It is important to review the calculated information to determine if the parameters are appropriate. Note that numerical value appearing below the yellow cell identifies the minimum size of the temporary storage tank necessary to allow the cistern to work appropriately. Here, the additional water usage is sufficient to justify a cistern, as long as an additional storage tank that stores approximately 1,703 gallons is also constructed to ensure the cistern is able to be emptied within a week.
## Results
| RESULT - 2ND STEP | |
| --- | --- |
| ADDITIONAL WATER USAGE IS SUFFICIENT TO JUSTIFY A CISTERN | |
| ADDITIONAL STORAGE TANK NECESSARY (GALLONS) | |
| 1,703.08 GALLONS | |
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