WHEN THE COASTAL Commissioners arrived in Los Osos to view the site of San Luis Obispo County's proposed wastewater treatment plant, they could not miss the signs planted in front yards, tacked onto lampposts, and held by people standing along the road: "This sewer is unsafe unfair unaffordable," "Less work for a better plan," and more. Two days later, on January 16, some 400 of the people behind those signs faced the Commission as it met to decide whether to permit the County to go ahead with its long-delayed project. To homeowners and renters in the unincorporated community of Los Osos/Bayview Park, the County's plan is tantamount to an eviction notice. They have a different proposal.       This community of 17,000 lies on the southern shore of Morro Bay, one of the few places on the coast where a young family or low-income person can still buy or rent a home. In 1988 the Regional Water Quality Board imposed a moratorium on water hookups, for the Morro Bay shellfisheries are suffering from nitrate pollution, which is attributed to septic tanks in Los Osos.       The County has wrestled with the problem for nearly 20 years and is now ready to build a tertiary wastewater treatment system of a type that was popular years ago, when federal funds met up to 85 percent of the costs. The trouble is, those funds are gone and the full costs of this project - estimated as $68.7 million by the County, $71.5 million by opponents - are to be met by property owners of this community, where almost 30 percent of the residents live in households with "low" or "very low" income, according to the 1990 State of California Census. The monthly charge will be $75-$80, says the County, $90 or more, say opponents - which, some residents joke, translates to "$5 a flush."       "The County plan addresses nitrates, but it would cost us our people. It will destroy them financially," Annie Mueller, president of the Los Osos Chamber of Commerce, told the Commission.       In August 1997, local citizens decided to go beyond protest. They pooled $17,000 of their own money, organized the Solutions Group, and enlisted scientists and engineers to craft a proposal for an Advanced Integrated Wastewater Pond System, which they believe would do the job for less while producing environmental benefits.       The expert team, working mostly pro bono, developed a version of the biological treatment system that has served the City of St. Helena for 35 years. It treats effluent in a series of engineered algae ponds, using photosynthesis and microbial fermentation. Pioneered and repeatedly refined by William Oswald, professor emeritus of engineering at the University of California, Berkeley, this system produces no sludge and results in clean water, said George Milanes, chief operating engineer for the St. Helena Water Resource Recovery and Treatment Plant.       Whether it would work for Los Osos, be cheaper and produce other benefits, is in dispute. The County's hydraulic planning engineer, George Gibson, believes the Solutions Group is "naive to the many rules and regulations," and that its plan does not conform to the Regional Board's requirements.       On January 16, the Coastal Commission voted unanimously to postpone a decision on the County's plant and directed the County to produce a comparative analysis by June.       The County submitted an analysis, but on June 8 the Commission found it inadequate and once again postponed a decision, to allow the County to provide more information.  - RG