Environmental design is one of the key approaches that DSD adopted for sustainable development. This chapter presents two examples of our recent works to illustrate how environmental improvement could be achieved at the design stage of our projects. The first case is on flood prevention, and the second one is about odour control in sewage treatment.
 

Environmental Design of the Sheung Wan Stormwater Pumping Station

The area in the vicinity of Wing Lok Street, Sheung Wan is low-lying and stormwater can back-flow causing flooding under extremely high tide. Flooding is common during heavy rain coupled with high tides. To alleviate the flooding problem, DSD has commenced construction of a stormwater pumping station at the waterfront in Sheung Wan to store and pump the runoff collected from the low-lying areas to the harbour.

Design Overview

The Sheung Wan Stormwater Pumping Station occupies an area of about 2 600 m2 on a waterfront site abutting Chung Kong Road. The pump house is the only above-ground structure comprising a 6 m high single-storey building with two levels of basement. The roof of this building will be garnished as a roof garden which will help to lower the temperature inside the plant room below. Sky windows will be installed at the roof level to introduce natural light into the interior of the pumping station.

It is expected that the incoming flows during dry weather from the upstream areas would be polluted. Therefore dry weather flow pumps will be provided inside the underground storage tank to pump polluted dry weather flows to a new sewer for conveyance to the sewage screening plant for treatment before discharging to the harbour.

For odour control, a deodorizer with air suction points at the mechanical screens and the low flow chamber will be installed to prevent odour from releasing to the adjacent park. Some of the energy saving features adopted in the electrical and mechanical design are summarized below:
The main pumps will be equipped with variable speed drives and harmonic filters. The pump control levels are designed to optimize motor running hours and thus the energy consumption varying with the pump motor speed. In addition, the variable speed drives impose lower motor starting current. The harmonic filters will maintain a high power quality and reduce energy loss.
   
Energy saving lamps such as T-5 fluorescent lamps fitted with electronic ballast will be provided inside the pump house. LED directional/exit signs which consume less energy will be used. Solar lightings shall be provided at the roof garden of the pumping station building. Built-in light sensor can turn on the light automatically at night to save energy.
Sheung Wan Stormwater Pumping Station
(Before works)
After works montage
 

Environmental Design of a Deodorising System at Sha Tin STW

Odour arising from the Sha Tin STW has become a growing concern when residential estates are developed in its vicinity. The closest residential development, Kam Tai Court in Ma On Shan, was completed in 2000. It has a population of more than 10 000 and is located immediately on the opposite bank of Shing Mun River.

To properly manage the odour issue at the Sha Tin STW, DSD has identified that the major odour sources are located at the inlet works, primary sedimentation tanks, and sludge treatment facilities. The major odour sources would be covered and the air would be treated to remove the odour.
 

Design Considerations of a Deodorization System

Biological odour treatment processes such as biofilter, bioscrubber and biotrickling filter offer the advantages of chemical and energy saving. Therefore, these processes have received much attention in recent years and have been developed for operation at a large scale STW. Biotricking becomes increasingly popular due to its reliable performance, lower life cycle cost and lower operation and maintenance requirements.

Schematic diagram of Biotrickling Filter Deodorization System

Microorganisms are provided in biotrickling filter with a suitable environment to stabilize malodourous compounds contained in the exhaust air stream when it passes through the filter. The microorganisms are attached to the air-permeable inert media. Water and nutrients are supplied regularly to sustain the growth of the biomass. Biotrickling filter can be designed to achieve 99 per cent removal of hydrogen sulphide at concentration up to 50 ppm. The biotrickling filter to be installed in Sha Tin STW will be able to handle foul airflow up to 50 000 m3 per hour.

In addition to saving chemicals and energy, the biotrickling filter offers a low operation and maintenance requirement over activated carbon adsorber and chemical scrubber. The activated carbon in an activated carbon adsorber needs to be replaced after a 6 to 12 month service period. Operation and maintenance requirements of chemical scrubber involve chemical supply management, and regular acid cleaning of scrubber packing to prevent scaling with calcium carbonate. The key maintenance requirement of a biotrickling filter is the replacement of media once every 10 years.

Biotrickling Filter Deodorization System under installation at inlet works of Sha Tin STW