Tuesday, September 14, 2010

Revisiting Pasir Ris

Thanks to Ria for her post on Pasir Ris water quality deteriorating due to the high concentration of enterococcus bacteria in the water.

An explanation of the situation from NEA press release (7 Sep 2010)
"According to results from a study released by NEA in 2009 during the last annual assessment of water quality at beaches, Pasir Ris beach’s water quality is affected by various possible sources, including minor leakage from older sewers, moored vessels, animals, as well as discharges from small-scale Sewage Treatment Plants (STPs) that presently serve the more remote areas in Pasir Ris. The low water currents in the concave part of Pasir Ris beach are not effective in diluting and dispersing the discharges."

I do not have the answers to most of the whys and hows in this issue. Instead, I will pose a few more questions. (Does this complicate the issue further?)

Fish farms in the vicinity
  1. I believe the staff on these farms dispose of their bodily waste directly into the sea straight without treatment so yes, this can be a source of the E. bacterium. (Anyone care to refute the part on direct disposal?)
  2. Fish waste is another issue altogether. Yes, they may decompose and cause the dissolved oxygen level to plummet but enterococcus??? Not likely as the bacteria normally comes from humans.
Aging sewage treatment plants (STP) in Changi and Tampines
  1. Can anyone throw some light on these plants? Where are they? Industrial (probably)? What kind of industry? What is in the sewage - human, animal or industrial waste?
  2. Doesn't sewage go into the PUB sewers?
  3. Why are they allowed to discharge unhealthy effluent into our waterways? Any discharge into a watercourse must be approved by NEA under the trade effluent regulations of Environmental Protection and Management Act.

    Unfortunately, under the same regulations, there is no mention on the limit of coliform, enterococcus or even bacteria for a discharge.

    The closest will be BOD5 (biochemical oxygen demand for 5 days) under the regulations. (In simple terms, BOD5 is a measure of the biodegradable substances e.g. faeces, urine, food in the water.)

    "The 5-day Biochemical Oxygen Demand at 20° Celsius (referred to in this paragraph as BOD) and the Chemical Oxygen Demand (referred to in this paragraph as COD) of any trade effluent analysed in accordance with regulation 11 shall not be in proportions greater than those set out below:
    (a) 50 milligrams per litre of BOD and 100 milligrams per litre of COD where the trade effluent is discharged into a watercourse other than a controlled watercourse"
    I assume the waterways which lead to the sea off Pasir Ris are non-controlled watercourse since they do not go into a reservoir. Still, 50mg/L of BOD5 is easily exceeded by normal domestic sewage (100 - 300mg/L). (Agricultural or food wastewater may go into the thousands in terms of BOD5.)

    How efficient are these STP in treating sewage i.e. percent reduction in BOD5? Did anyone check whether their discharges exceed the limits set out in the trade effluent regulations?

    Nevertheless, it is entirely possible that the discharge can pass the BOD5 test but yet contains enough human waste to give a high enterococcus count in the sea.
Sembawang Park

The water quality off Sembawang Park (also given in the same report) seems fine. Being located along the Johor Straits as is Pasir Ris Park, I imagine the seawater dispersion there is as dismal. Perhaps, the smaller number of fish farms and the lack of aging STPs and leaking sewers do make a difference.

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