Forensic Sewer Modeling — Using Advanced Modeling Tools to Bust Permit Violators and Anticipate Terrorist Attacks

With the advent of powerful sewer process modelling tools, the range of physical, chemical, and biological processes occurring within sewers can reliably be tracked across large sewer networks and over changing conditions. These processes include:
Heterotrophic Growth,
Particulate COD hydrolysis,
Fermentation,
Sulfate conversion to sulfide,
Carbonate cycle chemistry,
Sulfur cycle chemistry/biochemistry,
Denitrification, Sulfide precipitation by metals, Natural ventilation,
Air emissions, and
Sulfide corrosion of concrete.

Heretofore, sewer process modelling has focused on sulfide generation and fate for the purpose of mitigating problems related to odor and concrete corrosion. Recently, the development of powerful and complete sewer process modelling tools has expanded to include additional capabilities such as tracing wastewater plant upset conditions upstream to identify industrial discharge release events attributable to the plant upset and anticipating the fate, transport, emissions, and exposure to the public of chemical warfare agents within a sewer network. These modelling capabilities represent a new field of inquiry designated 'forensic process modelling'. Forensic process modelling implements sewer process modelling in combination with treatment plant process modelling and atmospheric dispersion modelling to address such challenges as busting industrial pre-treatment permit violators and averting terrorist attacks. This paper illuminates the topic of forensic process modelling by demonstrating its application in two case studies. Tracing an Acid Spill to its Source The Albuquerque Bernalillo County Water Utility Authority's main water reclamation facility faces challenges related to their lower effluent pH limit. In general, the problem is kept well in hand through careful dosing of magnesium hydroxide to the collection system which, in addition to improving plant effluent pH, augments sulfide control in the collection system. Recently, a sudden pH drop at the plant was detected and the drop appeared unrelated to any process or chemical in the control of the water authority. It was suspected that an industrial discharger caused the event. To test this hypothesis and identify the culprit, the Water Authority's WATS model (Wastewater Aerobic/anaerobic Transformations in Sewers), which was developed as part of their recent sewer odor and corrosion control master plan, was adapted to represent influent conditions corresponding to the measured pH upset. Then, each industry discharger was screened to identify those which deal with acidic waste streams. WATS model scenarios were then run to represent an acid spill from each of the short-listed industry dischargers. These scenarios allowed quantitative comparison of an acid spill which could cause the plant upset to the acid waste streams of potential culprits. WATS modelling scenarios resulted in identification of the industry discharger which could be attributed to the pH upset. This forensic inquiry culminated in actionable evidence toward enforcement of the violator causing the pH upset. Anticipating a Phosgene Attack Owners of civil infrastructure have long been charged with protection of public health and safety. Unfortunately, owner responsibilities have of late expanded to include anticipating and mitigating against terrorist attacks. In the case of water treatment plants, nuclear facilities, or other facilities enclosed within a defendable fence line, attack preparation may include target hardening and increased site security. But what of sewer networks for which there is no fence line and little separation between the public and the facility? Every household has anonymous access to the sewer system through its lateral connection. Also, the sewer headspace ventilates to the ambient atmosphere with serious implications for exposure to the public. The situation represents a uniquely intractable security problem in the face of a terrorist attack seeking to use the sewer network as delivery mechanism for a chemical weapon attack. DC Water in particular, due to its symbolically important location, must anticipate terrorism. Also, they own a number of odor control systems which force ventilate air from the sewer headspace to the ambient environment. For these reasons the trajectory of a chemical attack on their sewer network is of particular interest. In this case study, a phosgene attack was simulated using the WATS model. Phosgene is a chemical warfare agent which also has wide application in the plastics industry meaning that it is potentially obtainable by individuals (as opposed to foreign governments). Phosgene degrades the pulmonary tissues and victims may not be aware of receiving a lethal dose before it is too late. The DC Water WATS model, which was developed as part of their corrosion control and asset management program, was adapted to represent the chemical and physical properties of phosgene which determine fate and transport within a two phase (liquid stream and gas headspace) sewer network. The scenario tracks a 200-gallon release into a lateral clean-out connection upstream of one of the interceptor odor control stations. Dispersion modelling was used to estimate public exposure at a neighboring facility based on WATS model predicted concentrations from the odor control stack. The case study culminates on an assessment of the possibility of lethal exposure to the public based on this relatively small poison gas attack.