Constructing a prototype model of just about anything will save a lot of time and effort in the long run. As well as giving us a first insight into how our grey water system will work, the small scale model can be changed very easily when improvements need to be made.
A large part of our class time in Term 2 was devoted to building and testing the prototype of the pilot system we hope to have ready to install in the school grounds this coming summer. After researching on the internet the types of systems that have been used elsewhere in the world, we opted to develop a system that incorporates soil, rock and water-tolerant plants to clean the grey water. Just how well this can be done is the aim of our research, but at this point we just needed to show the basic engineering of our system was sound.
Blocked pipes, wet floors and we are on our way
A water container and three empty 10L paint tubs provided us with the equipment to hold the grey water system. The piping in the base of the containers was covered with river stone. and topped off with a mixture of shell rock and fines to complete the filter medium . Black alkathene piping and irrigation fittings enabled us to make the irrigation lines that connected the tubs.. After that it was pretty much making do with old drawers or whatever else could be found in the Science Department to create the staging and collect the water samples for testing . Gravity would do the rest.
Mr Ellwood in Horticulture kindly gave us some water marginal grass plants to get us started and the consensus within the group was that the plants would probably grow better if we planted them in compost on top of the shell rock. The tannins released from the compost, we would find out later, turned the filtered water the colour of whisky!
Things did not go immediately to plan when the grey water reservoir tap was turned on. The shell rock was a bit too fine and a bit too impermeable to handle even a just-dripping-tap which we soon found out about when we returned from a lunch break and had to clean up a huge puddle of compost and soapy shower water. The problem was traced to fines getting past our filters and into the piping system which began to block up over a very short period of time. Fortunately, we had designed the system so we could easily uncouple the tubing and clean out the pipes without the need to empty the containers. We have definitely taken on board the lessons we learned for the design of the outdoor system, particularly in how the particle size distribution of the filter medium affects the flow rate of the grey water . More of that though for another day.
What did we learn from our first attempt?
We believe we are pretty well set up to look at a whole set of water quality indicators when we start to measure the performance of our system. At this stage however, we have only begun to characterise the grey water system at the most basic level just to see if the cleaning system actually does anything to the waste water. Early indications are that it does.
You can see from the picture on the left that grey water is actually very grey and very cloudy. We had to dilute the grey water x10 to measure the turbidity (cloudiness) with our equipment and the value recorded was around 150 NTU's. which is high. What we now know is that most of this cloudiness is removed after one pass through our filters as the reading fell to 11 NTU's (cold tap water 1 NTU) The other noticeable thing about grey water is that it is fairly alkaline because of the soap in the water. We found out that the pH measure of this alkalinity is lowered to around the pH of the local tap water also after just one pass through the system.
We have plenty ideas about things we want to investigate over the coming weeks especially as we need to find out what plants are best suited to tolerate the wet, 'salty' conditions. We would like to see how much cleaning can be achieved and if it is possible, to harness the natural antimicrobial properties of native plants to improve the quality of waste grey water.