Town officials are advising residents that Tewksbury has switched to a triannual billing schedule for its water and sewer bills. The first bill will now be mailed in August and due in September, the second mailed in December and due in January, and the third mailed in March and due in April. Previously, the town billed twice a year. The first bill for this fiscal year was mailed last Monday and is due Sept. 26. Town Manager Richard Montuori said the new schedule will make the bills more manageable for residents and will improve cash flow for the town. He said the town had been exploring the change for several years and residents had requested it. The town initially looked at quarterly billing, but the dates would have coincided with property tax bills, making it more difficult for residents, Montuori said.