Inetgrated Planning for Wastewater Treatment and Recycling for a Small Community Development

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Courtesy of Water Environment Federation (WEF)

Integrated water resources planning was conducted for Olowalu Town, a proposed development in western Maui, Hawaii. Total water needs, which can be supplied by a combination of diverted surface water, well water, recycled water and captured stormwater, are estimated at between 1 and 2 mgd. The Olowalu development will generate approximately 0.5 million gallons per day (mgd) of municipal wastewater. That wastewater can be recycled for beneficial use and irrigate between 80 and 120 acres depending on the amount of storage provided and the type of water reuse.

A new development is being proposed by Olowalu Town, LLC in West Maui, Hawaii. The project site is approximately 4 miles south of Lahaina and comprises approximately 700 acres of the Olowalu area. The site was previously used as a sugar cane plantation.

Brown and Caldwell estimated that the potable water demand will be approximately 0.6 mgd. This value represents the typical drinking water quality demand for inside residential/commercial use and is based on 1,500 residential units with a per unit daily usage of 400 gallons. Typical nonpotable usage for outside water use, including irrigation, assumes 150 to 400 acres of irrigated area and a nonpotable demand ranging from 600 to 1,600 acre-ft/yr or 0.5 to 1.4 mgd (Brown and Caldwell, 2005).

A typical wastewater flow per capita in Maui is 137 gallons per day (gal/d). For the ultimate 1,500 residences and a typical 2.5 persons per residence, the daily wastewater generation would be approximately 500,000 gal/d or 0.5 mgd. For purposes of this evaluation an expected average daily flow of 0.5 mgd was used.

The water supply alternatives for the development include stream diversion and groundwater. Historically, the sugar cane plantation was irrigated with surface water diverted from Olowalu stream, which flows into the ocean and has cultural value as a flowing stream. Other sources of supply for nonpotable demands include recycled water and captured stormwater.

Olowalu is on the arid side of the island, so an integrated water supply is envisioned as being environmentally sound. The groundwater aquifer has at least a 3 mgd capacity and could supply the potable water demands of the project. The stream diversion used in the past has more than enough flow capacity for the potable needs but conflicts with the cultural value placed on a flowing stream as a fishery.

To minimize the diversion of stream flow, the potable supply would focus on groundwater while the nonpotable supply would be provided by a combination of recycled water and captured stormwater. Storage of recycled water and stormwater would be used to meet dry season nonpotable demands.

Wastewater will be managed onsite for individual residential lots in the low density areas provided that at least 0.25 acres are available on the lot. For medium density residential areas, the wastewater will be treated in interceptor tanks and collected using effluent sewers. Effluent sewers, primarily small diameter gravity sewers, will be utilized by following the terrain of the former plantation and having a minimal depth of burial. For this case, treatment of wastewater will occur in cluster systems using recirculating gravel filters, vertical flow wetlands, or biotextile filters, and then filtered through cloth filters, disinfected and reused locally.

Recirculating Gravel Filters
The recirculating gravel filter (RGF) evolved from the intermittent sand filter (ISF), in order to achieve better control of the dosing for higher rates. The filter medium usually consists of fine gravel. The RGF has been used for flow rates up to 1.0 mgd. The RGF is commonly used to treat septic tank effluent. They have also been used prior to ultraviolet (UV) disinfection for water reuse (Crites, et al., 1997).

The operation is similar to the ISF with the exception of a portion of the treated effluent which is returned to a recirculation tank where it is used to dilute the incoming effluent. By diluting the effluent higher application rates can be used. A schematic of a RGF is shown in Figure 1.

The RGF bed can be one third the size of an ISF for treating the same flow. An added expense comes from the additional recirculating tank. Although gravel media is more expensive than sand, the expected life for the media is longer and it will not clog from biological growth if properly designed and operated.

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