For the first time in nearly two decades, the Sonoma County Water Agency will re-examine how it charges customers for sewer service in eight small systems it operates.

The county board of supervisors this week agreed to pay Bartle Wells Associates of Berkeley about $129,000 for a year-long study of the way the costs of running these small sewer systems are divided among customers. The current formula for setting rates was established in 1995 when the agency took over operations.

Homeowners in these systems pay a flat annual fee, ranging from $738 in the Airport/Larkfield/Wikiup Sanitation Zone to $1,783 in the Occidental Sanitation District. Commercial customers and residents in smaller dwellings such as apartments are charged using a ratio reflecting how much water they are likely to use compared with a standard single family home. In most places, for example, an appliance repair shop or art gallery pays 68-82 percent of the local single-family-home rate on the assumption that neither business discharges much into the sewer.

A water-intensive business such as a restaurant, meanwhile, pays a much higher fee depending on its size. A 50-seat restaurant could pay as much as three times the residential fee in most systems, or up to 11 times in Occidental, where prices are high because the number of customers is small and sewer capacity is sharply limited.

The agency maintains a list of 44 categories of businesses and five smaller residential types, each with a specific ratio based on the single family home rate. Each of the eight systems has a unique schedule of ratios for those categories, with customers in tiny Occidental paying by far the largest amount, often many times more than equivalent businesses in other systems.

The study will examine whether the 1995 ratios still reflect the relative water usage rates of the various businesses and home types.

The consultant "will determine if this is really right," agency spokeswoman Ann DuBay said. "Is this still a good measure" of water usage?

In addition to Occidental and Airport, the systems in the study include the Russian River and South Park sanitation districts and the Geyserville, Penngrove, Sea Ranch Central and Sea Ranch North sanitation zones.

The agency also operates the Sonoma Valley Sanitation District, but most customers there pay in part based the actual volume of sewage they discharge, estimated by measuring their incoming water use during the winter months. Santa Rosa and other area sewer systems use a similar volume-based pricing for sewer service.

The eight small and rural districts run by the water agency cannot use volume-based pricing because customers tend to draw from wells, or buy water from elsewhere, and it is difficult to estimate accurately how much water they use monthly, DuBay said.

It's not clear whether the study will mean a change in the base fee paid by residential customers of each system. The water agency charges fees by simply dividing the actual cost of operating each system among the customers; the study may result in reshuffling the formula to allocate those costs in new ways, but it will not reduce the overall price of operating the systems, DuBay said.

(You can reach Staff Writer Sean Scully at 521-5313 or On Twitter @BeerCountry).