Greywater Reuse Systems

Filter Investigation

Aims
The aims of this investigation are:

1. To compare a range of filters for reduction in suspended solids (SS).
2. To compare different settling tank sizes for effectiveness in SS reduction.

Method and materials
Greywater from the laundry of a typical domestic residence was collected in a 580 L concrete tank, which was also set up as a settling or sedimentation tank. The volume of greywater collected and subsequently removed from the tank was 250 L per day. This was to simulate normal daily usage and a retention time of about 2 days.

Greywater was also diverted for collection and analysis via a ball valve tap positioned before the concrete tank. Suspended solids were determined for greywater that was passed through a number of different filters and this was compared to the input and output of the settling tank. The filters used were: 100 µm needlefelt, 200 µm needlefelt, standard nylon sock used in swimming pool filters, and a Zabel filter. This procedure was repeated a further two times. Only the initial wash water was collected and the subsequent rinse water was allowed to pass directly into the settling tank. It was assumed that the initial wash water would have a higher SS content than the rinse water.

Twenty litres of greywater was passed through each filter at a flow rate of about 1 L.s-1. Suspended solids were determined by passing 50 mL (unless noted) of greywater through a 0.45µm Whatman membrane filter (Cellulose nitrate). Triplicate measurements were taken. Each sample was oven dried at 104°C for twenty-four hours before mass was measured. Data for a larger settling tank was previously obtained as part of a PhD research thesis.

Results and Discussion

ANOVA and t tests were conducted on the raw data. Table 1 shows a summary of the data.

Percentage reduction in suspended solids with different filters.

The results shown in Table 1 are to be expected. Both the Zabel filter and the nylon sock are only coarse screen filters and finer particles would not be removed. The 200 µm needlefelt performed better than the 100 µm, and further investigation will need to be undertaken to examine this discrepancy. However, the settling tank was clearly the best in removing suspended solids. The wastewater in the settling tank was allowed to stand for twenty-four hours, without any addition of laundry water during this time. This approximates normal household use - one to three washing machine loads over a short period and then a day for the water to settle.

Statistical analysis has revealed that these results are significant, with p = 0.00025 comparing each filter system (ANOVA). T tests suggest that there is significant difference between the control (no filter) and the 100 µm and 200 µm needlefelt filters and the settling tank (p < 0.05 in all three cases). While the Zabel filter and nylon sock did reduce SS, these results are not significantly different from the control (p = 0.06).

Percentage reduction in suspended solids with large settling tank.

Average Inlet SS mg/L - 155
Average Outlet SS mg/L - 81
% Reduction - 48


The larger settling tank would have had ongoing input and, although the retention time would have been two to three days, the wastewater would still have high turbidity. Thus the percentage SS reduction is only 48%.

Conclusion
A number of different strategies are suitable for SS reduction. A smaller settling tank (e.g. 500 L, 0.9 m x 0.9 m) has been found to be very satisfactory for small greywater volumes from a single source or from a few sources, such as the laundry and/or bathroom. This may mean that a larger (septic tank size) sedimentation tank is not required for partial greywater reuse, but probably necessary for treatment of greywater for a whole household.

Filters provide another alternative to a settling tank. The Zabel filter is most suitable for bathroom greywater, where hair and soap flakes are common contaminants. The needlefelt filters are more suitable for laundry water, where lint and soil is the most common contaminants. A 200 µm filter is recommended as earlier trials with the smaller pore 100 µm filter required cleaning after about three weeks. Nylon socks, commonly used as swimming pool filters, are not appropriate for greywater treatment.

Reference
Mars, R. (2001). Using the Submergent Triglochin huegelii for Domestic Greywater Treatment. PhD Thesis. Murdoch University.