Proceedings

GPS-X Modelling to Optimise Nitrification and Risk Assess Design Proposals – Case Study of Brockhampton Sewage Treatment Works

Ruswa, E.1 and Copp, J.2 Chadha, M3.
1Severn Trent Water Process Design Engineering (UK) 
2Primodal (Canada),
3Severn Trent Water Asset Creation (UK),

(free)

Abstract:

Brockhampton STW is currently being re-permitted to a tighter ammonia limit of 1.2mgN/l from 5mgN/l. Compounding the issue was the fact that the works was experiencing unexplained daily ammonia spikes that were risking compliance and increasing OPEX requirements. To diagnose the issue and investigate future options, Severn-Trent Process Design Engineering commissioned Primodal to develop a GPS-X process model. Model development showed the measured effluent ammonia data was well represented if a significant delay was included in the aeration system sub-model. This assumption was confirmed when the Brockhampton (15,000PE) aeration control code was found to contain a hard-coded 30-minute delay and found to be an exact copy of the Minworth STW (1,500,000PE) control code. The modelling work predicted that eliminating the delay would eliminate the ammonia spikes. The delay was removed on-site and the spikes were eliminated. The model has subsequently been used to risk assess future scenarios and this has negated the requirement for extensive civil works in the current capital scheme.

Keywords:
GPS-X, Modelling, ASP, Anoxic, Aeration, Nitrification, Risk assess

Introduction
In AMP6, a number of sewage discharge permits are likely to be tightened as a result of the Water Framework Directive. River modelling undertaken by Severn Trent Water has identified that Brockhampton STW is subject to an ammonia permit of 1.2 mg/l (95 percentile).

Table 1:            Design Basis

Current (2013) Future (2028)
Population Equivalent 12150 16025
Permit [BOD/SS/NH3/P/Fe/Al] (mg/l) 15/25/5 10/25/1.2
Permitted DWF (m3/d & l/s) 3360 & 38.9 4220 & 48.8
Permitted FFT (m3/d & l/s) 8208 & 95 9072 & 105
Measured DWF (Q80) (m3/d & l/s) 3333 & 38.6 4018 & 46.5
Measured average flow (m3/d & l/s) 4752 & 55.0 5435 & 62.9

The present works consists of an inlet pumping station (Archimedes screw, no overflow), primary treatment (4 Dortmund tanks) followed by a 6 pocket ASP, and four Finals Settlement Tanks. There was a history of unexplained daily NH3 spikes in the final effluent at Brockhampton which have significantly increased the operator visits to the site over the past few years.

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