Outfall design

OUTFALL DESIGN

An outfall is an underwater pipeline that discharges wastewater into receiving water. Even if the wastewater has received extensive treatment, it is desirable to dispose the effluent to minimize possible impairment to quality of receiving waters near point of discharge by using a diffuser. 

A diffuser is a section of the outfall, usually deepest and most seaward portion with small holes or ports along its length. These ports discharge wastewater in small quantities as opposed to the entire flow being discharged at one point.

Diffuser ports may be simple holes in the outfall pipe or short tubes extending from the pipe. The type of port used depends upon particular conditions of final installation. The various elements of outfall design process are:

• Site selection
• Outfall hydraulics
• Dilution and Mixing
• Diffuser port design
• Pipe design
• Pipe support systems
• Construction methods

The design is phased into four elements:

1. Feasibility study
2. Pre-design
3. Preliminary design and
4. Final design

The flow diagram illustrating how various design components and phases are integrated is depicted in the chart below:

Site selection:

• Discharge port for an outfall should be located in proximity to its companion wastewater treatment plant.
• First step in outfall design is to determine feasibility of linking treatment plant to nearest receiving water.
• This involves study of:
• Topographic maps
• Hydrographic charts
• Literature of coastal processes like tides, waves, currents and geology
• Characteristics of the site and obtaining related water quality data related to proposed receiving water
• If the information suggests that, from the construction cost and water quality point of view one or more outfall routes is feasible, the pre-design engineering studies are commenced to select a final route or an alternative disposal system must be considered.

The following outfall siting criteria are considered in detail in the pre-design stage:

• Bottom topography and turf zone
• Physical oceanography
• Water quality and
• Wastewater soils and geology

A hydrographic survey of the general area of the proposed outfall alignment is performed using an electronic recording echo-sounding device combined with horizontal positioning system. Helicopters may be used as sounding platforms. Subsequently, profiles of bottom topography are plotted. Analysis of profiles will reveal potential outfall routes. It is desirable to have a continually declining grade for the outfall line. This prevents potential sludge build-up in low parts and accumulation of air at high points which can reduce the hydraulic capacity of the outfall. There is no ideal slope for locating the diffuser. A relatively flat slope is desirable. The diffuser port should be designed such that there should be equal discharge from  each side of the diffuser. This results in equal dilution. A mild slope is beneficial for achieving uniform port discharge. 

Penetrating the surf zone is most expensive and difficult phase of constructing an ocean outfall because of continuous wave attack in shallow water.

Sporadic storms....