This filtering cistern has a layered filter bed above a perforated collecting pipe and lateral branches. Water passes downward through the bed and is drawn out through the pipe network. The illustration follows Hiscox’s items 575–576. The calculator separately compares entered volume, flow and rectangular plan dimensions; it does not predict water quality.
Filtering Cistern Interactive Calculator
Enter chamber volume, inflow, length and width to calculate nominal volume/flow time, plan area, mean depth and flow per plan area.
Equation Used
- Positive entered flow, volume, length and width.
- Mean depth assumes that the entered volume occupies the rectangular plan area.
- Nominal V/Q time does not equal actual hydraulic residence-time distribution.
- No filtration efficiency, disinfection, potability, clogging, overflow or structural model.
Results describe the entered geometry and nominal volume/flow time. They do not predict treatment performance or drinking-water quality.
Filter bed and collecting pipes
The pump pipe runs down the cistern wall and across its floor, with perforated lateral branches. Hiscox describes a gravel layer over the pipes, then sand, charcoal and a final sand layer. The section shows water moving down through these layers before entering the pipework; the plan view shows how the branches spread beneath the bed.
The moving markers show water-flow direction only. Filtration rate, pump suction, contaminant capture and media resistance are not calculated from the four inputs.
Compare chamber size and inflow
Enter the flow and chamber volume from the case being evaluated. Their quotient is a nominal volume/flow time. Entered length and width give plan area; combining that area with volume gives a rectangular mean-depth equivalent and flow per plan area.
These quantities can organize a design discussion, but they do not establish whether a target contaminant will settle or whether stored water is suitable for any use.
Volume, flow and plan geometry
Nominal volume/flow time is V/Q. Plan area is A=LW. With volume converted from litres to cubic metres, mean depth is (V/1000)/A. Flow per plan area is Q/A, and nominal volume turnovers per hour are 60Q/V.
The required settling conditions depend on particle properties and water movement. Neither a target velocity nor an acceptable detention time can be determined from these four inputs alone.
Example entered chamber
For 3,000 L at 100 L/min, nominal V/Q time is 30 minutes. A 3 m by 1.5 m plan is 4.5 m², giving a volume-derived mean depth of about 0.67 m and flow per plan area of about 22.2 L/(min·m²). These figures do not state removal efficiency.
Modern inlet pretreatment and water quality
EPA rainwater-harvesting guidance supports inlet screening and pretreatment to reduce debris and sediment entering storage. EPA's baffle-box fact sheet describes screened settling chambers separated by baffles, with accumulated water passing over a baffle into the next chamber. These arrangements separate debris and sediment from the storage path; their performance depends on the particular water and installation.
For an actual cistern, assess rainfall and overflow routing, first-flush arrangements, the particles to be removed and water movement through the chambers. Provide suitable maintenance access, exclusion of pests and structural support, and meet the water-quality requirements for the intended use.
Questions
Is V/Q the actual residence time?
It is a nominal volume/flow ratio. Real residence times vary because of mixing, dead zones and short-circuiting.
Does the filter bed make water potable?
No. The illustration and volume/flow calculation do not establish microbiological or chemical safety.
Does the calculator predict sediment removal?
No. It lacks particle and hydraulic performance data.
References
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances (1901), page 155, items 575–576: filtering cistern plan and section. The EPA references below concern modern inlet pretreatment, not the construction of the historical filter bed.
U.S. EPA, NPDES Compliance Inspection Manual, rainwater harvesting pretreatment: inlet screening, first-flush diversion and settling pretreatment.
U.S. EPA, Storm Water Technology Fact Sheet: Baffle Boxes: screened sediment chambers separated by baffles and overflow into the next chamber.
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