One potentially untapped source of biomass might be grass clippings. It the tanks could be small enough to be operated at the household level, then perhaps one

One potentially untapped source of biomass might be grass clippings. It the tanks could be small enough to be operated at the household level, then perhaps one enterprising neighbor could buy the neighborhood detritus or offer to convert it for them. Even if it's not competitive price-wise there is significant value for (some) people in feeling environmental; burning carbon negative gas in your car from your yard would probably fit that category.