Methods of treatment

Two methods of treatment can be considered:

1. On-site treatment and disposal; and

2. Off-site treatment and disposal.

Where on-site treatment is to be undertaken consideration should be given to the following:

1. Where large volumes of effluents are produced and/or different types of contaminants, large equipment areas may be required. Sufficient space must also be allowed for maintenance and inspection of such equipment.

2. Settlement/storage areas for effluent need to be sized not just for average flow but also for peak periods. Where production is based on a shift system, peak flows created during holiday periods (shutdown, major maintenance, etc.) should be considered.

3. Where effluents require primary, secondary and possibly additional tertiary treatment, attention should be paid to the various treatment processes with regard to personnel safety and public sensitivity to on-site treatment.

4. Where concentrated alkali and/or acids are stored and used on-site as part of the treatment process, care should be exercised to prevent misuse, fire, and security and health hazards. The provision of emergency showers, eyewash stations, etc. needs careful consideration.

5. If equipment malfunctions during the treatment process, adequate precautions should be taken to prevent the discharge of untreated effluent. Such precautions should be the provision of emergency collection tasks or the use of approved, licensed effluent-disposal traders.

6. Where accidental discharge of untreated effluent does occur, the appropriate water authority and/or environmental health officer should be advised immediately. All steps should be taken to limit the extent and intensity of any potential contamination.

7. Where small and/or single contaminant effluents are encountered, packaged treatment plants may be acceptable. Consideration should, however, be given to capital cost, payback period, reliability of equipment, maintenance, plant-life expectancy and contaminant-removal efficiencies.

8. Pipework material for conveying effluent to treatment plants should exhibit resilience to corrosive attack by the effluent as well as scouring and erosion created by the material content of the effluent.

9. Consideration should be given to plant operation in a shift system and any requirements for an analyst to be present during operational/non-operational periods.

10. Precautions must be taken against freezing for external pipework, tanks, meters, gauges, and monitoring equipment.

11. Assessments should be made for electrically operated process equipment that may require an essential power supply in the event of a main failure.

12. The quality of the effluent discharge must be regularly checked. Depending on the quantity and type of discharge, this may require an in-house laboratory and analysis room.

13. The water authority may limit the quantity of final treated effluent, and monitoring of the final out-fall may have to be considered in conjunction with a holding tank.

14. Large or small on-site treatment plants will create sludge concentrates that require disposal. Where large quantities of sludge occur, on-site de-watering filters may be considered with dry sludge cakes properly removed from site by licensed contractors. Alternatively, small quantities of wet sludge concentrates may be removed and disposed of by similar contractors.

Where off-site treatment is undertaken the following should be considered:

1. Cost comparison with on-site treatment.

2. Availability of approved, licensed contractors to handle the type of effluents being considered.

3. Reliability of licensed contractors during emergency, weekends and holiday periods.

4. Space requirements for holding untreated effluent prior to removal from site.

5. Accessibility, safety and security associated with the holding vessels by the vehicles of the licensed contractors.

6. Suitable pumps may be required to pump from holding tanks into licensed contractor vehicles.

4.3.2 Site drainage

The discharge of surface water from a site may originate from three potential sources: rainwater from building(s), surface-water runoff from paved/hard standing areas and subsoil drainage (groundwater)

1. The rainwater runoff from buildings depends on the geographical location and storm-return period specified. Rainwater runoff from a roof is relatively clean and can discharge directly to a watercourse, lake, etc. without passing through an interceptor.

2. The surface water runoff from paved/hard standing areas also depends on rainfall intensity calculated from the geographical locations of the site and storm-return period. However, the return period for a site will be far higher than for a building in order to ensure prevention of persistent flooding of the site. In many instances the local authority may specify the storm-return period as the design criterion.

Where development of a greenfield site or an extension to an existing building takes place, the rate of storm water runoff may necessitate the provision of a balancing pond or reservoir.

While the drainage design may be able to cater for minimal surcharging, any substantial rise in floodwater can be contained by the balancing pond and minimize flooding to the site and damage to plant ecology. Lining of a balance pond must be considered to prevent water seepage. Suitable linings are clay or butyl rubber sheeting.

Where potential flooding to the site is minimal an alternative to the balance pond could be an open-trench system that could provide on-site storage and added security to watercourses.

The open-trench system provides easy maintenance and may obviate the need for highway curbs and gullies, but requires adequate security to prevent vehicle collision.

Surface water from hard standing areas subject to gasoline and oil contamination (e.g. car parks) must pass through an interceptor prior to discharge to a watercourse, surface-water drain, etc. The sizing of gasoline/oil interceptors varies between local, state and regional areas. Many agree that the first 5 minutes of surface-water runoff is the most contaminated, and accept a reduced interceptor size, while allowing a higher flow rate, caused by increased storm intensity, to bypass the interceptor chamber (see Figures 4.1 and 4.2).

Discharge of effluent and general site drainage 4/23

3. Subsoil drainage may be required for the following reasons:

(a) Seasonal fluctuations in water-table level may cause isolated flooding. Subsoil drainage may be considered to keep the water-table level relatively constant.

(b) Where underground springs occur, layering of subsoil drainage may be considered to maintain water-level equilibrium within the subsoil to permit building construction or similar activities.

(c) Draining of permanently flood land such as marsh or bog land.

Gravity or pumped discharge

Wherever practical, surface-water runoff should be designed for gravity discharge, preferably located at the lowest part of the site. However, where a gravity system is impractical or impossible a pumped discharge must be considered.

When sizing a pumped discharge the following should be evaluated:

1. Pumps, valve controls, etc. to be in duplicate;

2. Maximum discharge permitted into watercourse by water authority;

Section A-A

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Figure 4.1 Traditional three-chamber gasoline interceptor. (Dimensions are in millimeters.) All pipes within the chambers through which liquid passes should be of iron or another equally robust petrol-resistant material


Figure 4.1 Traditional three-chamber gasoline interceptor. (Dimensions are in millimeters.) All pipes within the chambers through which liquid passes should be of iron or another equally robust petrol-resistant material

Figure 4.2 An alternative two-chamber gasoline interceptor. (Ventilation not shown: dimensions are in millimeters)


Figure 4.2 An alternative two-chamber gasoline interceptor. (Ventilation not shown: dimensions are in millimeters)

3. Discharge duty to be twice dry weather flow subject to (2) above;

4. Will essential power supply be required to both pumps?

5. Space requirements for pumps, including installation, maintenance and inspection.

Figures 4.3-4.7 show edge and bank details for storm-retention reservoirs.

50-100 mm limestone rubble in gabion mattress

50-100 mm limestone rubble in gabion mattress

0.5 mm 'Fibretex' sheet Subsoil

0.5 mm 'Fibretex' sheet Subsoil

30-50 mm overlaps at joints Secti0n


95% polypropylene and 5% polyester 170 g/m2

Laid vertically on slopes Figure 4.3 Rip-rap bank (Fibretex/stone)

150 mm armour layer or larger boulders or stone

150 mm armour layer or larger boulders or stone

and sewn or stapled Figure 4.4 Rip-rap bank (Terram/stone)
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