An expansion of the city’s HPOWER waste-to-energy plant in Campbell Industrial Park is expected to begin sending electricity into the Hawaiian Electric Co. grid by September, officials announced today.

The city is adding a third boiler to the plant, which will expand its generating capacity by nearly 60 percent to 73 megawatts from 46 megawatts. The expansion will allow the plant to increase the amount of garbage it burns to 3,060 tons a day from 2,160 tons a day.

Because of the expansion HECO is required to update its agreement to buy electricity from the city, a change that is subject to approval by the Public Utilities Commission. HECO and the city have asked the PUC to issue a ruling by Sept. 1.

Under the proposed power purchase agreement HECO will pay the city from 11 cents to 21 cents a kilowatt hour for the electricity generated by HPOWER. That includes a capacity payment of 5 cents a kilowatt hour for electricity produced between 7 a.m. and 9 p.m.

The HPOWER plant, which went online in 2000, is operated for the city by New Jersey-based Covanta Energy.