The City of Houston has selected planning, engineering and program management firm Lockwood, Andrews & Newnam Inc. (LAN) to develop...
Colorado treatment plant chooses new monitoring technology
The Littleton/Englewood Wastewater Treatment Plant in Englewood, Colo., has recently implemented processes to effectively monitor ammonia levels in wastewater. In addition, the plant now is able to effectively monitor prechlorination, dissolved air flotation thickeners and dissolved oxygen (DO) concentration. Accurate measurement of these multiple parameters is imperative to ensure a timely and cost-efficient process.
The need for efficient wastewater treatment technologies is becoming progressively more pressing due to the ever-increasing population, urbanization and industrialization of society. For this reason, large treatment plants are required to adhere to strict wastewater regulations. The effective monitoring of various applications, including ammonia, is vital to the wastewater industry and is essential to ensure that wastewater treatment plants are able to operate efficiently while protecting the environment.
Ammonia is a common concern for the wastewater industry, as it can cause severe problems in the disinfection process of water and, in extreme cases, can cause drinking water to become hazardous. Ammonia also is fatal to fish and water organisms when it is changed to unionized ammonia gas in increased river temperatures. In 1994, ammonia limits were initiated by the Colorado Department of Public Health and Environment (CDPHE). The CDPHE specifies an individual monthly average and daily limit to which treatment plants must adhere. WWTPs must therefore keep their discharge within these limits in order to retain their permits. Ammonia monitoring can be complex and costly; therefore, a solution is needed to facilitate the process.
Seeking Effective Techniques
Founded in 1977 as a pure oxygen-activated sludge plant, the Littleton/Englewood WWTP is the third-largest publicly owned treatment works in the state of Colorado. The plant receives sewage from Littleton and Englewood, as well as 21 smaller districts located in the service area. In 2009, a $110-million construction project was completed, transforming the plant into a 50-million-gal-per-day trickle filter/solids contact facility with post-nitrification and post-denitrification capabilities. The large advanced treatment plant is required to exceed secondary treatment requirements and remove ammonia from wastewater, working in compliance with local, state and federal regulations.
Water from the treatment plant discharges into the South Platte River, which is widely used for recreational purposes and drinking water, and supports various ecosystems in surrounding areas. The river is classified as a Class 1 Recreation Scheme and contains many species of fish, including trout. Due to the danger ammonia can cause fish and water organisms, among other concerns, the continued monitoring of ammonia levels in the water is vital. The plant also required solutions for a number of other applications including monitoring of prechlorination dissolved air flotation thickeners and DO concentration.
A Multisensor Solution
In order to ensure accurate measurement of these parameters, the Littleton/Englewood plant set up a procedure to find the new instrumentation it needed. The DO models previously used were becoming increasingly outdated and were no longer suitable for use in controlling DO levels in the aeration basins.
Various products were assessed for their suitability in monitoring oxygen levels while increasing productivity, and each type was tested side by side.
The IQ Sensor Net system from Xylem Analytics provided the most accurate results and involved the least amount of operator intervention. The system allows the plant to customize its analytical measurements and enables the facility to form any combination of up to 20 sensors into one sensor network that communicates directly with the plant’s internal computerized control system.
The wastewater treatment plant now utilizes an advanced system solution to manage its multisensor network, which allows the facility to keep growing, adding more sensors as needed. Implementation of the system at the facility has enabled the plant to undertake a wide variety of different applications using a single system.
Littleton/Englewood now is able to take on more applications, including:
Centrate return ammonia. By measuring the amount of ammonia in the aeration basin, the team is able to monitor the contribution of ammonia from the return of anaerobic sludge dewatering centrate to the aeration basin. An ammonia concentration target is entered into the SCADA system and the centrate pump output is varied to attain the desired concentration in the aeration basin. By monitoring and controlling the amount of ammonia present, the ammonia loading to the nitrification process is kept within target ranges, which improves efficiency in the conversion of ammonia into nitrate.
Prechlorination disinfection. One of the most important advantages of the new technology for the plant are the benefits that arise during disinfection. Wastewater chlorination provides the disinfection necessary to protect the receiving stream and downstream users. A certain ratio of ammonia to chlorine must be maintained for this process to work most efficiently.
The ammonia concentration prior to disinfection is monitored using the selected sensor, with the value being fed into the SCADA system and compared to the desired ammonia concentration setpoint. The collated information is used to vary the output of pumps that can bypass a flow of ammonia-rich secondary effluent around the nitrification process, thereby maintaining the desired ammonia setpoint and maintaining the critical ammonia to chlorine ratio. The technology ensures that the chlorination process works and guarantees that the plant achieves low fecal coliform levels while also keeping ammonia levels at a minimum.
DO monitoring. To assure that carbonaceous biochemical oxygen demand is fully converted in the aeration basins, a DO setpoint of 1.5 mg/L was established. If any values exceed the setpoint, electricity is wasted through over-aeration. If the values are below the setpoint, conditions for the formation of unwanted filamentous bacteria are encountered. For this reason, accurate measurement of both high and low oxygen is required in order to ensure the appropriate treatment process. The sensor utilized by Littleton/Englewood prevents signal disturbances due to rising air bubbles, eliminates false readings and improves stability—especially important for aeration tanks.
Dissolved air flotation thickeners. Prior to introducing new instrumentation, the plant used a constant dosage of polymer to keep the subnatant suspended solids at an acceptable level when thickening waste-activated sludge and primary sludge in dissolved air flotation thickeners. This resulted in periods of under-dosage, both when suspended solids were higher than desired and when polymer was being wasted. The suspended solids in the subnatant are returned to the treatment process, adding to the loading on those processes.
Therefore, it is vital to use a reliable online instrument to ensure an efficient and energy-saving control of the process. The subnatant suspended solids are measured and the values are fed into the SCADA system. The values are compared to the setpoint and polymer adjustments are made.
The versatility of the multisensor system has allowed Littleton/Englewood to do a lot more using a single system, resulting not only in significant cost savings, but also in enabling multiple probes to be used in a variety of locations throughout the plant. The facility also has been able to automate sections of its treatment procedures based on the information that has been collated by the system, significantly accelerating processes.
Due to the location’s weather conditions, the plant previously was unable to use probes that required water to be pumped from an outdoor location to an indoor analyzer, as it was highly likely it would freeze in the winter. The new multisensor system does not have this problem and can continue to function outside in freezing conditions.
Prior to its use of the multisensor system, the plant used equipment that required a lot of man hours to ensure it was running correctly. Since the introduction of the new technology, the plant has benefited from the instruments working with less assistance from staff. Whereas data once had to be manually retrieved from the field, it now comes in through the sensor system automatically. The information that is collected from the various probes can be analyzed for trends and patterns. This can be used to learn more about the finer details of the operation and further increase long-term productivity and efficiency.