"tank Vac" a Revolution in Rain Water Tank Cleaning

Abstract: This paper describes a new rain water tank cleaning system called "Tank Vac" which aids the regular removal of debris and sludge from the bottom of water storage tanks. The system relies on overflowing water from the bottom of the tank rather than the top using a unique siphon system. When the tank is full a valve in the overflow line is triggered to allow water from the bottom of the tank to flow out the overflow pipe at a rate high enough to produce a full siphon. The magnitude of the flow creates a cleaning action at the base of the tank. After a short flush period a ball cock allows air into the line to break the siphon and the cleaning action ceases until another high rainfall period. "Tank Vac" installations in household rainwater tanks have decreased E.coli levels at the bottom of the tank from 450 counts per 100ml to zero after a 4 month period and increased oxygen concentration levels markedly.

Keywords: Rainwater harvesting, Water tank cleaning, Siphonic flow, Environmental engineering

1 INTRODUCTION

Many people in rural and urban New Zealand collect and store rainwater to augment other water sources. The practice was essential in earlier times, declined with urbanization and the formation of centralized water supplies, but has experienced a rebirth as local city and dis-trict council's look to solve water supply and water run off problems for their growing communities. One such council is the Rodney District Council in the Auckland Region which has experienced rapid population growth in the last 20 years, and is in desperate need of a new reservoir to meet growing water demand.

Usable water can be obtained from various sources, namely streams, ponds, rivers, lakes, and wells, but the most pure source is the sky through rain. All sources gain their water sup-ply from rain essentially, and from natural col-lection rain water flows into streams, then to rivers and lakes and finally to the sea. Along the way the once pure water can become con-taminated and undesirable to use without ex-tensive purification.

With industrial development there comes a time when the marginal cost of the next quan-tity of water to purify at a centralized facility is expensive to the point that distributed collec-tion at individual homes is a viable economic alternative. This is the situation that has arisen in Rodney District Council. Economic motiva-tion is driving councils to consider rate reduc-tion incentives for households installing rainwater tanks to supplement central water supply.

It makes sense, if it is possible to do in a clean and efficient manner, to catch the rain and us-ing it where it falls. This approach to water supply is known as "Rain Harvesting" and as a technology house holders and the communities are discovering many benefits.

Rainwater storage is a good standby in times of emergency, such as during power outages or when the water table lowers and wells go dry. Rainwater is soft and doesn't cause buildup of scale in water heaters, plumbing lines or household humidifiers. It does not compromise the effectiveness of pesticides when mixed in spray rigs. Rainwater does not stain laundry and plumbing fixtures (no iron or manganese). And it is better for landscape watering than waters that are naturally alkaline or rich in sodium.

2 EXISTING RAINWATER TANK SYSTEMS

Rainwater harvesting technology is simple to implement and essentially uses existing facili-ties on a house, such as the roof, guttering, down pipes, combined with leaf diverters, first flush diverters and a storage tank. Water for use is taken from about 300 to 400mm off the bottom of the tank and an overflow pipe is usually located at the top of the tank to avoid water build up in the roof down pipes.

Figure 1. Conventional rainwater tank

With conventional rainwater tanks (Figure 1) contaminants that are not diverted by either the leaf diverter or the first flush diverter, enter and settle to the bottom of the tank. Over time sludge builds up and becomes a breeding ground for E.coli and other bacteria, and is a settling point for parasites in cyst form such as G.lamblia (giardia). As a consequence dis-solved oxygen levels in the water reduce and general water quality at or near the bottom of the tank becomes poor (see Appendix). Over-time the height of the contaminated water re-gion in the tank rises and when it reaches the takeoff pipe the householders know it's time to clean the tank because the water discolours and smells. The tank is cleaned manually by draining the whole tank via a valve at the bot-tom of the tank or through a suction arrange-ment if the tank is underground.

This approach to maintaining household water quality is not satisfactory and has been a major deterrent