Rainwater Collection for a sustainable future.
As we see and face the impact of climate change in our present world, both government and non-goverment entities strive to make the necessary ways in order to mitigate environmental issues and concerns which contributes to climate change.
Different strategies have been made to combat climate change. In the building design and construction industry, Green Building Codes have been made not only to promote but also to require developers and building owners to implement ways to build sustainable buildings and communities.
In Davao City, a rainwater ordinance have been developed and implemented which require even residential buildings to provide a rainwater catchment system.
The intent of this article is to dig deeper into ways a rainwater harvesting system can be designed not only to get code compliance but also to really achieve the intent of the code: to design with sustainability in mind.
What is Rainwater Catchment System?
First and foremost, it is important to define and delineate the rainwater catchment system from any other drainage system in the building and development. A rainwater catchment system is the same as rainwater harvesting system. For the sake of this article, we will use the term Rainwater Catchment System or RCS for short.
ARCSA/ASPE/ANSI 63-2020: Rainwater Catchment Systems defined Rainwater Catchment System as: Water system for collecting and utilizing rainwater, consisting of a cistern(s), pipe, fittings, pumps, and/or other plumbing appurtenances, required for and/or used to harvest and distribute rainwater. Also called rainwater harvesting system or rainwater collection system.
The most common system that is usually interchangeably thought of as an RCS is the Stormwater Catchment System. Though we knew that both of them are basically coming from the same source which is rainwater, these two systems are different. An RCS is a system where rainwater are collected from catchment areas such as roof and roof decks of buildings in where pollutants are minimal and therefore requires minimal filtration. Unlike RCS, a stormwater drainage and detention systems includes run-off from roads, sidewalks, walkways, parks & playgrounds, and any other catchment surfaces. The water collected from a stormwater contains a lot of sediments, organic pollutants, and potentially bacteria and viruses which requires a ton of filtration if it is to be stored and safely re-used for non-potable water use.
What are the components of a Rainwater Catchment System?
Rainwater catchment systems is basically subdivided into the following sub-system components:
Catchment System - These are the catchment areas which collect the rainwater such as Roofs and roof decks.
Conveyance System - A conveyance system is composed of the gutters and/or piping networks for the collection of rainwater from the catchment catchment system.
Filtration System - Depending on the use of the collected rainwater, different filtration system is needed before it is stored, and when it is used. A more detailed talk about rainwater filtration is discussed below.
Cistern or Rainwater Storage - The biggest component of an RCS is its rainwater storage. The rainwater storage tank is sized based on the following considerations:
Projected Rainwater Collection
Rainwater Usage Demand
Minimum requirement based on Code.
Rainwater Distribution System - If rainwater is not used, there will be no point of catching it. Therefore, it is important that all rainwater catchment systems provide a way for the efficient usage of collected rainwater. This system may belong to the water distribution system of the building and is composed of pumps and piping networks.
How to determine the Storage Volume of a Rainwater Storage Tanks?
One of the most identified component of an RCS is the Rainwater Storage. This is what is commonly required by codes and have a major impact on the cost of the system. How then is the rainwater storage tank volume derived? Basically, there are 3 ways to determine the volume required for an RCS:
Code-based approach - this approach is the most commonly known way because it is what is required in order to get a building permit. The following are some of the code references with regard to rainwater catchment systems:
Code | Developed by | Description/Remarks |
The Philippine Green Building Code | DPWH | Providing a basic minimum rainwater storage requirement based on Gross Floor Area of the building. Applicable only to large developments covered by the PGBC requirements. |
Ordance No. 0298-09: Davao City Rainwater Ordinance of 2009 and its Implementing Rules and Regulations | Davao City | A code in Davao City which requires new buildings to provide a rainwater catchment system. |
ARCSA/ASPE/ANSI 63-2020: Rainwater Catchment Systems | ARCSA and ASPE | A comprehensive document and design guide to Rainwater Catchment System |
Suppy-Base Approach - This approach looks at the available supply based on precipitation data and design the cistern capacity based on that limitation.
Demand-Based Approach - This calculation is based on the non-potable water requirement of the building or its users. It is calculated by obtaining the number and use profile of building occupants.
In most cases, an individual method of calculation would not be enough. For example, when you design a cistern capacity for a given project, you would need to know the available supply based on local average precipitation data. You would then need to determine the demand of non-potable water use in order to know if the rainwater source would be sufficient to cover most of the demand. Lastly, you need to determine if the cistern capacity you calculated would pass the minimum code requirement, then make the necessary adjustments.
It would not be reasonable to provide a very big cistern if the precipitation data is very low, or if the non-potable water use is very minimal, unless it is code required.
Rainwater Filtration: Is it required?
Most rain is safe to drink and may be even cleaner than some public water systems. Rainwater is only as clean as its container. Unless you directly collect it from the falling rain, then its perfectly fine to drink. Once rain touches on the roofs and other parts of the building, it will need additional treatment before it is safe to drink. Roof surfaces collects air-borne dusts which could carry bacteria and viruses. Roof surfaces also could be contaminated with bird’s droppings. Some catchment areas near trees collects leaves and other organic contaminants which usually build up in the gutters.
A water filtration system is required in order for the rainwater to be drinkable. A sediment filtration is enough though if it is to be used only for toilet flushing, landscape irrigation, and other non-potable use. Without sediment filtration, sediment particles would block parts of the valves and fixture components.
Rainwater Pre-filtration Systems:
Rainwater pre-filtration is needed in order to maintain a clean water within the tank. Without any pre-filtration, the tank would be filled with silts and organic components such as leaves and grass and small branches which would rot within the tank and would lead to the development of bacterial growth within the tank.
There are 2 common types of pre-filtration:
Screen Filtration - This rainwater pre-filtration is composed of a device with a mesh screen to remove large particles, leaves, and small branches. There are different designs of rainwater pre-filter in the market. The most common rainwater pre-filter which is available in the Philippines is the Leaf Eater by Blue Mountain Co.
A Leaf Eater by Blue Mountain Co. Rainwater First-Flush Diverter - This device aims to divert fine particles and contaminated water from the tank by diverting the first flush of rain and preventing it from going into the tank. The first flush diverter is slowly drained to be ready for the next rainfall.
Working principle of a First Flush Diverter
Rainwater Post-Filtration
Post Filtration refers to the filtration of rainwater after it is being stored. Post filtration need varies depending on the usage of the rainwater. Rainwater tank with direct faucet outlets for landscape irrigation and other non-potable uses would not need additional filtration. However, if you would need to use the collected rainwater using a separate distribution network for toilet flushing, it is recommended to use a sediment filter to eliminate any particulate matter which would cause blocked along stop valves and fill inlets of water closets.
When using rainwater for potable use and shower, it is essential to treat the water well using a multi-stage filtration system. In this set-up, It is important to inform the client of their responsibility of regular maintenance such as inspection and cleaning of the tank, cleaning and replacement of cartridges of filters, and making sure that the filtration system is working properly.
A Multi-Stage Filtration System usually consists of the following components:
A sedimentation filter to remove small particulate matter such as silts and sands.
An Activated Carbon Filter chemicals and organic compounds in the water. Carbon Filters also effectively remove odor from the water.
UV Filter to kill bacteria and viruses which are able to passed through the earlier filtration stages.
For an enhance filtration for readily drinkable water, a Reverse Osmosis Filtration system is recommended to be installed at the bottom of the kitchen sinks which will then provide a safer clean water for drinking from its potable water faucet outlets.
In implimenting a filtration system, it is important that the owner is well informed of the regular maintenance requirements of each specific systems in order for the system to provide a safe water suitable for its use.
Author:
RMP James Ray Magdadaro
Registered Master Plumber (PRC Reg. No. 6491)
Managing Partner - MEP Global Co.
Principal Architect - JLM ARCHITECTS
Contact: jlm@jlmarchstudio.com
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