Hawaiian Dredging Construction Company

A breath of fresh air┬áUpgrading HawaiiÔÇÖs largest wastewater treatment plant is no small chore, which is why Hawaiian Dredging Construction Company has been called in for its expertise, Gary Toushek learns. The Sand Island Wastewater Treatment Plant, owned and operated by the City of Honolulu, is the largest treatment plant in Hawaii and handles the discharge of industrial and residential wastewater from the city of Honolulu as well as the resort area of Waikiki.  Sand Island is at the entrance to Honolulu Harbor and contains mainly industrial enterprises, as well as shipping containers being loaded and unloaded to and from freight ships. The average capacity of this treatment plant is 90 million gallons of effluent per day (mgd), with peak flows in excess of 150 mgd. The plant has been cited by the US Environmental Protection Agency (EPA) as requiring upgrades in terms of requirements set out in the Clean Water Act, and one of the related problems is odor control due mainly to hydrogen sulfide gas, a by-product of sewage decomposition. Stepping up to meet the challenge is Hawaiian Dredging Construction Company (HDCC), founded in 1902 to dredge the main channel into the Pearl Harbor Naval Base. The companyÔÇÖs dredging also reshaped HawaiiÔÇÖs other harbors, as well as waterways on the outer islands. Marshy coastal areas in Honolulu, and especially Waikiki, were reclaimed with dredged fillÔÇöeventually 5,000 acres of land were reclaimed by HDCCÔÇÖs work. Then general contracting overtook dredging projects during the 1920s, and since then HDCC has built many of HawaiiÔÇÖs resort hotels, high-rise condominiums, office buildings, retail malls, highways and other civil projects, as well as waterfront and industrial installations. Kurt Hara is senior project manager of HDCCÔÇÖs wastewater division, one of the companyÔÇÖs five major divisions, and one of the two industrial divisions (the other is power generation). With a degree in mechanical engineering, heÔÇÖs been with the company for nearly 20 years, rising through its ranks from construction engineer to project engineer, then superintendent, and then to his current position. HeÔÇÖs responsible for the Sand Island Wastewater Treatment Plant upgrade to its odor control system because of the major scope of the project. ÔÇ£This upgrade began in May 2007 and was scheduled to take two years,ÔÇØ Hara says, ÔÇ£but there have been delays due to changes that the city has made, as well as other political considerations, and the upgrade project is only half complete so far. The EPA has mandated secondary treatment, and up to now the plant has been operating on a discharge permit that allows primary treatment. The size, capacity and current loading will test the implementation of this secondary treatment. Our task is to install technology that cleans the air quality, the bad odor emanating from the plant due to hydrogen sulfide emissions being well above the acceptable level.ÔÇØ The new odor control systems are being installed in three strategic areas of the plant. The first is the headworks, which is the first interface for the incoming flow of effluent material, with an airflow rate of 40,000 cubic feet per minute (cfm). The second is the primary clarification stage, where solids are settled and removed from the flow, with an airflow rate of 50,000 cfm. The third area is known as ÔÇ£the solids,ÔÇØ where solids are further processed before sending them to a privately developed operation to produce fertilizer or landfill, with an airflow rate of 30,000 cfm. According to the EPA, all biological odor control scrubber installations are required to treat hydrogen sulfide, the major component of the foul air, at a maximum expected rate of 300 parts per million (ppm) by volume, to be reduced to 3 ppmÔÇöan effective removal of 98 percent of the hydrogen sulfide. To put this in perspective, a typically safe working atmosphere is monitored for a hydrogen sulfide concentration at a maximum level of 10 ppm.ÔÇ£Our situation calls for an added area or site with two towers for temporary odor control at the clarifiers before the permanent clarification odor control system is operational,ÔÇØ Hara says. ÔÇ£The infrastructure work for these areas includes new electrical serviceÔÇöone new electrical building for the primary clarification odor control system, one retrofitted electrical room for the headworks and solids odor control system, as well as new underground water and drain lines. The three primary sites or areas will implement a biological odor control system that essentially uses bacteria in a synthetic medium with a consistency similar to steel wool, with a lot of surface area to attach itself to, enabling it to digest the hydrogen sulfide. This project also includes the refurbishment of the treatment plantÔÇÖs four gravity thickener tanks, which is where the primary sludge is further thickened. This work initially established the projectÔÇÖs critical path of construction, since only one tank could be refurbished at a time.ÔÇØHaraÔÇÖs experience recalls that initially, and typically several decades ago in a large treatment plant like this, a chemical scrubber was used as a primary system to capture the sulphur, which was expensive at that time and not very effective. A carbon secondary system was used to polish or clean up the liquid that emerged from the primary system. ÔÇ£In smaller installations, carbon is usually sufficient, but with a higher hydrogen sulfide installation, so much carbon is used that it becomes expensive to change out. The major problem is that the treatment plant is processing so much industrial waste as well as residential waste, and when it rains heavily and the sewers carry more run-off, it adds to the volume of liquid material to be processed.ÔÇØHara thinks the treatment plant has reached a critical point that puts it in a difficult situation, because of the antiquated nature of most of the equipment and transfer lines and its inability to easily handle peaks of capacity. ÔÇ£It requires alternative options, such as a new transmission line and pump stations, but the treatment plant itself canÔÇÖt be taken out of service for redirection. One consideration is a valve trickling system that works vertically, and from what I hear, that may the direction theyÔÇÖre headed.ÔÇØ ÔÇô Editorial research by Jason Moore┬á