Hello Tom - This is certainly a valid point. We decided to delve into some of the documentation we have on the subject and also did a bit of Internet research to check this out properly.
According to the CAT (
Centre for Alternative Technology) tipsheet we have, there seems to be no problem with what you're doing. The people at CAT, however, recommend pouring the water (from washing up water - which you don't use) onto the soil only, not onto the plants.
As you say, the water contains nutrients for the plants, but can also contain solid particles like lint and hair (definitely the case for clothes washing and bath water) which may clog things up. So what you're doing is probably the simplest and easiest solution. Since we wanted to irrigate our little garden via gravity-fed pipes with holes, however, we needed to filter out anything (including suspended solids) which could clog up the system. Since we already needed a bin to store the warm water while it cools (we have a solar hot water system so we use pre-heated water for washing), it wasn't a big effort to convert it into a sand filter.
Wayne Brownlee from
Free Water Systems warns that the phosphates contained in some detergents - while being beneficial for the garden in some respects - can be damaging to fynbos and other specialised plants such as conifers and roses over time. They can also influence the soil's alkalinity and he suggests to switch to biodegradable detergents which contain less phosphates. We do have proteas and a pincushions which are irrigated with this water and they both seem to be thriving thus far. We do, however, also try to use more eco-friendly washing powders. The use of a sand filter is recommended by the Gaia-Movement Booklet No 6 to treat greywater so it can be used for
general use, taking out organic matter, but leaving in nitrogen and phosphate, so our sandfilter won't help with phosphates. The
Wikipedia article on Greywater also mentions that most powdered detergents, and some liquid detergents, are sodium-based, which can inhibit seed-germination and destroy the structure of clay soils.
The excellent and very entertaining page about common
greywater mistakes and preferred practices gives a wealth of information on what not to do. They also suggest only using sand filters for cases where water without suspended solids is needed (e.g. for drip irrigation or perforated pipes), and otherwise rather to design the simplest system possible, so you don't end up undoing all the environmental benefits of reusing water.
Thank you for the great comment and keep on using your greywater just the way you are!
