Consultant's Corner: First-Of-Its-Kind Solution Revolutionizing Wastewater Treatment

Full-Scale Implementation of a Biological and Chemically Enhanced High-Rate Clarification Solution for the Treatment of Wet Weather Flows

By Julian Sandino, CH2M HILL

North Texas Municipal Water District (NTMWD) serves 1.7 million customers by operating one water treatment plant and four regional wastewater treatment plants including the Wilson Creek Regional Wastewater Treatment Plant (RWWTP). This facility discharges into Lake Lavon, which also serves as the district’s water supply.  With continued growth of the service area and the increased challenge of wet weather flow management, NTMWD evaluated several alternatives and selected to upgrade and expand the Wilson Creek RWWTP.  One key characteristic of the selected alternative was the incorporation of dual-purpose components to provide both advanced treatment of dry weather flows as well as treatment of wet weather flows components aimed at further protecting Lake Lavon as a water supply source. 

Upgraded Process Configuration

The treatment approach uses high-rate clarification (HRC) in a dual-function mode to achieve both tertiary treatment during dry weather conditions while also providing additional secondary clarification capacity to a reconfigured activated sludge process during wet weather events.  A high-rate ballasted flocculation system (Actiflo) is used for the tertiary application, and is converted to a secondary clarifier configuration (BioActiflo) during wet weather events to provide additional biomass separation capacity for the activated sludge process during the high flow conditions (see Figure 1).  During wet weather events, the plant’s biological nutrient removal (BNR) activated sludge process is reconfigured to a step feed mode to treat part of the diluted excess flows that are being diverted around the primary clarifiers. In the BioActiflo mode, return activated sludge (RAS) from the plant’s secondary treatment process is routed to a short contact time aerated basin where it blends with excess wet weather flows.

Figure 1. Reconfiguring a high-rate clarification system from tertiary treatment to a biological and chemically enhanced high-rate process for wet weather flow management.

This mixture of biomass and influent wastewater is then treated through the high-rate chemically enhanced clarification facilities (using ferric chloride as coagulant and polymer as a flocculation aid) working in parallel to the plant’s conventional secondary clarifiers. This innovative approach increases Wilson Creek’s dry weather treatment capacity from 48 mgd to 64 mgd and meets a future anticipated P effluent limit of 0.1 mg/l and reduces TOC levels, while providing biological treatment for wet weather flows up to 168 mgd.

Bench-scale testing of the biological and chemically enhanced HRC process was conducted as part of the design of the recommended facilities. Test results suggested that the system could be designed to provide the 85% BOD₅ minimum removal efficiencies as required in the plant’s NPDES permit by providing aerated contact times in excess of 15 minutes for the wet weather flows and the biomass in the RAS flow. Tests also indicated that the chemical dosages required were similar to those typically observed in other more conventional wet weather flow applications of Actiflo, even though the influent solids concentration was now considerably larger due to the addition of biomass. A summary of the high rate clarification upgraded facilities is presented in Table 1.

Dynamic Simulation Of Wet Weather Operations

Dynamic simulation of the proposed configuration using Biowin (Envirosim version 3.1.0.833) was also undertaken as part of the design effort in order to quantify the impact of removing wet weather loads in the BioActiflo process, which would primarily be the increase in mixed liquor suspended solids (MLSS) in the plant’s biological reactors that could potentially overload the existing conventional secondary clarifiers. The simulation was performed using an assumed peak storm event hydrograph derived from modeling of the plant’s contributing collection system. 

Simulation results indicated that the MLSS may increase by 10 - 20% (e.g. from 3400 mg/L to 3700-4000 mg/L) for the design peak storm event depending when the wet weather loading “flush” occurs: if the flush occurs before change-over to wet weather mode, the MLSS increase will be less due to capture in the primary clarifiers.  For most storms, which will be less intense than the design storm and consequently BioActiflo mode will operate for less time, the increase in MLSS will be less. Figure 1 shows the result of the simulation assuming the first-flush occurring before the changing over from the tertiary configuration to the wet wet-weather mode.

The simulation effort also allowed for the establishment of a daily incremental increase in wasting (approximately 10% per day) to determine the length of time it may be necessary to recover the original MLSS concentration.  For the design peak storm event, where the MLSS increases by up to 10-20%, increased wasting may be necessary for 7-10 days.  For most storms, which will be less intense than the design storm, the length of time to recover will be shorter, i.e. 1-7 days.

Figure 2. SRT and MLSS for scenario 1 (wet weather flush arrives before change-over to BioActiflo mode).

This wet weather approach provides secondary treatment by providing biological treatment of all flows and is the first full-scale facility in the world being permitted for the BioActiflo process. The No Feasible Alternatives Evaluation described in USEPA’s 2005 Draft Policy on Peak Wet Weather Flow Diversions and the 2009 Draft Guidance on Preparing a Utility Analysis should not be triggered with this treatment approach because secondary treatment is being provided for all flows entering the facility. This project has set precedence on how regulators approach wet weather treatment requirements at wastewater treatment facilities nationwide. They are doing startup of these facility improvements as this article is being published. This is a first of its kind in the world!