Here half of the length of 3M AP902 membrane

I was using Diamond POE filter but after a while the water pressure gets lower which I think is because of the cloth/membrane so I had a stainless steel outdoor sand filter installed 4 years ago.  Under my kitchen sink I have a double housing 1 micron PP filter connected to my drinking water dispenser (0.001 micron) and kitchen faucet. Another PP filter is installed for my washing machine.
I paid about RM2100 for the outdoor filter, the PP filters for the kitchen and washing machine and the RO water dispenser. The water dispenser uses Omnipure filters and Filmtec membrane. I think there are USA made.

Maintenance wise,  I change/service the outdoor filter every 3 years and it cost me about RM350.00 each time. The PP filter is RM9.00 each, which I change every 3 to 4 months. There are many PP filters in the market some as cheap as RM10.00 for 4 pcs. to RM20.00 per pc. The PP filters that I am using is 140g which is harder. The water dispenser has 3 filters and 1 membrane. the 3 filters need to be changed every year and it cost me RM250 for 3 omnipure filters whereas the membrane is changed every two years at RM240.00. S it works out to be RM 540 per year..

-Please comment if the maintenance cost is acceptable. Should I change the filtration system?