Estuary Processes and Managment - River Dynamics and Integrated River Management - page 524

Several stages of HATS are being planned. In Stage 2A, the outfall will be used to carry additional
sewage flows as more urban areas are connected to the sewerage system (Fig. 8.67), and the Chemically
Enhanced Primary Treatment (CEPT) effluent will undergo chlorination. The dry weather flow will increase
to 32 m 3 /s. In Stage 2B, secondary treatment will also be provided with a view to reducing the organic
pollutant concentrations by 90%. Table 8.5 summarizes the treatment level, discharge flows, ammonia
nitrogen concentration, and cost of the three different stages of HATS.

T able 8.5 Stages of Harbor Area Treatment Scheme (HATS) in Hong Kong
Sewage flow

Effluent NH 3 conc.
(mg/L)

Cost
(US$)

Stage Treatment level

( M m 3 /d) (m 3 /s)

1 1.6 18.5 CEPT (advanced primary) 19.0 1.09 billion

2A 2.8 32.4 CEPT + disinfection 19.0 1.14 billion

2B 2.8 32.4 CEPT + biological + disinfection 1.90 1.52 billion

A three-dimensional hydrodynamic model with boundary-fitted curvilinear grid (8 uniform vertical
layers for the present study) covering the entire Hong Kong Waters and Pearl River Estuary has been
developed and calibrated to simulate the tidal circulation of Hong Kong Waters. The effect of effluent
discharged from the HATS outfall have been computed for a typical 1 day spring-neap cycle using open
boundary conditions derived from field data for ķ the dry season; and ĸ the wet season. The HATS
diffuser is represented by six 8 port rosette jet groups.
The Water Quality Objective (WQO) for NH 3 in Victoria Harbor is 0.021 mg/L; it can be used to define
the boundary of the instantaneous mixing zone. Under different tidal stages and ambient density structures,
the buoyant jets discharged from the sewage outfall may be trapped at different levels. Hence, the extent
of the mixing zone also varies quite significantly with time. The predicted mixing zones (with NH 3 as the
target pollutant) may be quite different under flood and ebb tide conditions. For environmental assessment
purposes, it is more convenient to define a mixing zone in statistical terms. With the computed concentration
time series for the simulated period, we can determine the 95 percentile of the concentrations at each grid
cell. The mixing zone can then be meaningfully defined in terms of the 95 percentile concentrations and the
WQO. The zone obtained will indicate the region outside which the WQO will be met 95% of the time.
Figure 8.68 shows the mixing zone obtained for NH 3 in the wet season with HATS Stage 2A discharge. It
can be seen that the mixing zone defined by unionized ammonia extends well into the western harbor
towards Lantau Island.
E. coli is another key water quality indicator; the effluent concentration can be on the order of 10 7
counts/100mL. With initial dilution on the order of 100, the E. coli concentration at the end of the near
field will be on the order of 10 5 counts/100mL—which is significantly greater than the regulatory allowable
level of 180 counts/100 mL. Figure 8.69 shows the predicted mixing zone using E. coli as the target
pollutant for the wet season under flood conditions. Even in the presence of bacteria decay, the extent of
instantaneous impacted zones is much greater than that for the un-ionized ammonia, NH 3 .
Environmental risks are best expressed in statistical terms. The methods of predicting the impact of
waste discharges outlined in this chapter enables scientific determination of risks. An integrated risk
assessment methodology has recently been proposed to quantify the ecological risks of HATS based on
the target pollutant (Choi et al., 2009). The environmental risk is defined in terms of a hazard quotient
(HQ); a HQ > 1 means that the organism is exposed to a pollutant concentration higher than the threshold
value above which the marine organisms may possibly die. It is found that the probability of HQ > 1 for
HATS Stage 1 is 0.11 for the wet season but just about 0.06 for the dry season. The risk increases to
0.08-0.13 with additional sewage loads of 0.8 million m 3 /day at the same level of treatment (HATS