You might look into Bio-sand filters. They can remove all kinds of pollutants and would be a great non energy intensive first step in water purification.

"Turbidity

Results for turbidity reductions vary depending on the turbidity of the influent water. Water with high turbidity looks dirty due to sand, silt and clay floating in the water[3]. Feed turbidity in one study ranged from 1.86 to 3.9 NTU. In a study water was obtained from sample taps of water treatment plants from three local reservoirs. It poured through a slow sand filter and results showed that turbidity decreased to a mean of 1.45 NTU [2]. In another study using surface water a 93% reduction in turbidity was observed [8]. As the biofilm above the sand ripens turbidity removal increases [2]. Although biosand filters do exhibit a high reduction in turbidity, slow sand filters show higher removals due to a slower filtration rate [2].
[edit] Heavy Metals

There is limited research on removal of heavy metals by biosand filters. In a study conducted in South Africa, the filter was found to have about 64% removal of iron and 5% removal of magnesium[8].
[edit] Bacteria

In laboratory studies, the biosand filter has been found to have about a 98-99% removal of bacteria [8]. Over a time period of about two months it was found that the biosand filter may increase in the removal for E. coli due to biofilm formation. The removal after this time period ranged from 97-99.99% removal depending on the daily charge volume and percent feed water amended with primary effluent to the filter daily. The addition of primary effluent or waste water facilitates the growth of the biolfilm which aids bacterial die-off [2]. Research in the field shows that implemented biosand filters reduce fewer bacteria than ones in a controlled environment. In research conducted in 55 households of Bonao, Dominican Republic, the average E. coli reduction was found to be about 93 percent[9].
[edit] Viruses

Lab tests have shown that while the reduction of E. coli from these filters is quite significant, the attenuation in viruses is significantly less due to their small size. In a study using to bactiophages the removal ranged between 85% and 95% after 45 days of usage [2]. A recent study has suggested that virus removal increases significantly over time, reaching 99.99% after a period of approximately 150 days[10].
[edit] Protozoa

In one lab test the biosand filter also got greater than a 99.9% removal of protozoa. In tests for one type of protozoa, Giardia lambia had a 100% removal for 29 days of usage. Another protozoa, Criptosporidium oocysts, showed to have a slightly lower reduction (99.98%) possibly due to their smaller size. This removal showed to be comparable to that of the slow sand filter[11]."

http://en.wikipedia.org/wiki/BioSand_Filter