Septic Installation & Cesspool Conversion in Hilo

We install septic systems and convert cesspools across Hilo and the coast just north of it, about 8 miles north of our Keaau base. Hilo is different from most of the island because it has a real sewer system. It just doesn’t reach everyone, and the homes it misses are the ones we work on.

Hilo at a Glance

  • 44,186 residents and 17,732 housing units in the Hilo census area in 2020. The whole South Hilo district had 51,551 residents.
  • About 120 inches of rain a year at Hilo International Airport (1991 to 2020 normal), with measurable rain on about 273 days. DOH notes heavier rain upslope.
  • One county treatment plant, in Keaukaha, serving about 30,000 residents and treating an average of about 3 million gallons a day.
  • About 8,700 cesspools in the Hilo Bay priority area named by the Department of Health in 2017.
  • 27 Mauna Loa lava flows have been identified in and near Hilo, ranging from the 1880 to 1881 flow to flows more than 24,000 years old.

Sewer in Hilo: Who Has It and Who Doesn’t

The county’s Wastewater Division runs 120 miles of sewer mains, 16 pump stations and seven treatment facilities on the island. The Hilo plant in Keaukaha was built in the early 1990s and serves about 30,000 people. In 2020 the Hilo census area alone counted more than 44,000. The numbers don’t line up exactly, since the plant’s service area and the census boundary aren’t the same, but the gap shows that a lot of Hilo homes are not connected.

The plant itself is in the middle of a major rebuild. In March 2024 the county signed an administrative consent order with the EPA after inspectors found serious problems there, and an upgraded plant has to be running by June 30, 2035. A $337 million first phase broke ground in 2025 and is scheduled to finish in 2030. In June 2024, about 607,000 gallons of effluent were accidentally discharged into Hilo Bay.

We can’t tell you from here whether a particular street is on the sewer, and we won’t guess. The county Department of Environmental Management can confirm whether a main runs past your lot and if you’re required to connect. If no main is available, your options are a septic system, an aerobic treatment unit, or a cesspool you’ll have to replace by 2050.

Why Hilo Bay Made the Priority List

The DOH’s 2017 cesspool report rated the Hilo Bay area Priority 3, for impact on sensitive waters. It listed about 8,700 cesspools across 31 square miles, discharging about 5.6 million gallons a day. The report’s reasoning is specific to Hilo. Heavy rain upslope sends a lot of stream flow and groundwater into the bay, and the breakwater slows its mixing with the open ocean. A UH Hilo study cited in the report found elevated nutrients and fecal indicator bacteria in the bay.

The Hilo Bay label doesn’t come with its own deadline; Hilo homeowners face the same 2050 cutoff in Act 125 as the rest of Hawaii. What can speed things up is a permit that adds bedrooms, or a cesspool that needs pumping more than twice in a year. Our Act 125 explainer covers how the law works.

Old Lava, Thin Soil, Constant Rain

USGS geologists put it plainly: Hilo is built entirely on lava flows from Mauna Loa’s northeast rift zone, and most of them are older than 1852. North of the Wailuku River, those Mauna Loa flows lie over older ash and Mauna Kea flows. The biggest single flow in the area, the Panaewa flow, erupted roughly 1,500 years ago. It forms the shoreline from Waiakea Pond out to Leleiwi Point, and Hilo’s airport sits on it. USGS notes that without it, Hilo Bay wouldn’t exist.

Older doesn’t mean deep. Federal soil surveys of the low windward slopes around Hilo mostly describe a thin layer of organic matter and volcanic ash over pahoehoe or aa. Where that layer is measured in inches, rock shows up well above the depth a leach field needs. Each neighborhood page below has the soil details for that area.

Here’s what that means for a Hilo system:

  • Rock work is normal. Fractured lava means a soil replacement bed built from imported fill, sitting a minimum of 3 feet above groundwater.
  • Rain sets the schedule. With rain on most days, a trench can fill with water overnight. We plan digging, inspection and backfill to keep open-trench time short.
  • Drainage matters. In older neighborhoods, how roof runoff and yard drainage move around the treatment bed is part of the design.

Typical statewide conversion prices run $20,000 to $50,000, and heavy rock pushes a Hilo job toward the high end. See our conversion cost guide for the breakdown.

Hilo’s Housing Stock

Hilo’s older neighborhoods were built over many decades, and the work they need varies:

  • Older homes on cesspools outside the sewer service area, often on small city lots where setbacks are tight.
  • Homes changing hands. Selling a Hilo house with a cesspool is legal, yet buyers and their lenders will want to know the plan. Our buying and selling guide covers that.
  • Remodels that add bedrooms, including ohana units, which give DOH the option to require the cesspool be replaced.
  • Larger lots upslope and to the south, where new construction needs an engineered system from the start.

On small lots, setbacks decide the layout: 5 feet from buildings and property lines, 9 feet from the property line for a seepage pit, and 50 feet from streams. Hilo has a lot of streams.

Hilo Neighborhoods

Each of these has its own page.

  • Waiakea was the main settlement of the Hilo district in 1823. Today it reaches inland toward Waiakea Uka, and USGS dates the pahoehoe at the Waiakea Homesteads to about 9,500 years old.
  • Kaumana climbs the slope above town. Kaumana Cave lies in the 1880 to 1881 Mauna Loa flow, the youngest lava in the Hilo area, which is less than 150 years old.
  • Keaukaha is home to the county treatment plant. A University of Hawaii study released in 2025 found sewage reaching Keaukaha’s groundwater at about 10 times the rate previously estimated, from both the plant’s outflow and local cesspools.
  • Panaewa lies along Highway 11 between town and Keaau. Much of it is Hawaiian Home Lands farm and residential leases, where DHHL approval comes before any septic work.

North of Hilo

North of the Wailuku River, Highway 19 follows the old sugar coast. Small towns sit along the gulches, and the county has named a few of them for future sewer work.

  • Wainaku, Paukaa and Papaikou start just past town. Papaikou, once home to the Onomea Sugar Company, had 1,166 residents in 2020. Wainaku had 1,147 and Paukaa had 434.
  • Pepeekeo, about 7 miles north of Hilo, had 1,826 residents in 2020. In 2025 the mayor listed Papaikou and Pepeekeo among the next communities in line for county wastewater projects, after Kealakehe, Pahala and Naalehu. No schedule was given.
  • Honomu is the small town on the road up to Akaka Falls State Park, with 452 residents in 2020.
  • Hakalau, about 15 miles north of Hilo, was a plantation town until its mill shut down in 1974.

South Toward Puna

Past Panaewa, Highway 11 crosses into the Puna District at Keaau, where Mauna Loa lava gives way to Kilauea lava. Puna has no county sewer at all, so the work there looks different. See our Puna septic services overview or the Keaau page.

Starting a Hilo Project

The first thing to settle is whether a sewer main is available to your lot. If it isn’t, the next questions are how much soil sits over the rock, how close the streams and property lines are, and how many bedrooms the system has to serve. Your engineer handles design and the perc test, DOH reviews the drawings, and our crew handles the excavation and installation. Request a free estimate

Frequently Asked Questions

How do I find out if my Hilo home is on the county sewer?

Ask the county Department of Environmental Management. A county sewer bill is a strong clue, but only the county can confirm whether a main runs past your lot.

My house drains toward Hilo Bay. Am I on a shorter clock?

No. The bay was ranked for its effect on sensitive waters, not given its own deadline, so 2050 still applies. A bedroom addition or a cesspool that keeps failing can bring DOH in earlier.

Is the treatment plant upgrade a sign that sewer service is expanding in Hilo?

The work under way is mainly about repairing and modernizing the existing plant under the EPA order. County officials have named Papaikou and Pepeekeo among future sewer projects, but none has a published schedule. Don’t count on a new connection unless the county confirms one for your street.

Why do Hilo installations take extra planning in wet months?

Hilo gets rain on roughly three days out of four. Open excavations take on water fast, so we stage materials, schedule the engineer’s inspection and backfill quickly to keep trenches from sitting open.

My Hilo lot is small. Will a septic system fit?

Often, but setbacks decide it. The system has to stay 5 feet from buildings and property lines and 50 feet from any stream. Lots recorded before August 1991 aren’t held to the 10,000 square foot minimum, which helps many older Hilo parcels.

Are there loans for Hilo homeowners?

The USDA keeps an office in Hilo that handles Section 504 home repair help. Eligibility turns on household income and on whether your address counts as rural, so ask them directly. A new state loan program under Act 204 is due around the end of 2026.