Dawson Sewage Treatment Plant

Environment Protection Licence #2531

A requirement of the Dawson sewage treatment plant Environment Protection Licence (#2531) is to meet concentration limits for specific water quality parameters from treated effluent prior to discharge.

The Dawson licence has a range of concentration limits listed including: 50 percentile, 90 percentile and 100 percentile limits. These limits relate to the monitoring of results throughout the licence period.

To meet a 90 percentile limit, the monitoring results must not exceed the specified concentration limit for 90% of the time during the monitoring period. Likewise, 50 percentile concentration limits are to be achieved for 50% of the results throughout the monitoring period. The 100 percentile limits are not to be exceeded, alternatively the maximum concentration.

The Dawson licence monitoring period is reported upon annually with an anniversary date of the 9th of September. As a condition of the licence the treated effluent is monitored monthly prior to discharge (EPA I.D. point No 4).

Discharge concentration limits are specified for biological oxygen demand (BOD), total suspended solids (TSS), ammonia and pH. The corresponding concentration limits stated in the licence for each of these parameters are listed in Table 1. Oil and grease is also monitored monthly. There is no specific concentration limit for this parameter.

During wet weather there is provision for overflow discharge of treated effluent to Browns Creek (EPA I.D. No.1) and Dawson River (EPA I.D. No.2). One sample per overflow event for 50% of the overflow events in the reporting period are required for biological oxygen demand, oil and grease and total suspended solids at points 1 and 2. The addition of monitoring faecal coliforms at point 2 is also required. There are no specific concentration limits for these discharges.  

 Dawson STP Licence monitoring results:

Table 1. Dawson licence effluent concentration limits for Biological Oxygen Demand, Total Suspended Solids, Ammonia and pH.
Dawson STP
EPA I.D point No 4.
Discharge to waters
Effluent quality monitoring
BOD
TSS
Ammonia
pH
mg/L
mg/L
mg/L
50%ile
Limit
20
20
90%ile
Limit
30
30
5
100%ile
Limit
35
35
10
6.5-8.5