New Biological Phosphorus Removal Concept Successfully Apllied in a T-Ditch Process Wastewater Treatment Plant

Luofang Wastewater Treatment Plant adopted a new concept of enhanced biological phosphorus removal in combination with a traditional Triple Ditch (T-Ditch) process to optimize the efficiency of biological phosphorus removal. The new concept included the addition of a thickener and anaerobic cell ahead of the T-Ditch system. Recorded data shows that the modified system is operating successfully with effluent which is much better than most T-Ditch systems. It also shows that it has better treatment efficiency than other complicated biological treatment processes with the same wastewater characteristics and same temperature conditions.

Luofang Wastewater Treatment Plant is located in Shenzhen, China. The original plant was built in 1995 utilizing a normal two-stage activated sludge (AB) process that was designed for 100,000 m3/day of treatment capacity. The Modified University Cape Town (MUCT) process concept was used in the second stage of the two-stage system. In 1998, an additional 250,000 m3/day of treatment capacity was added. The new plant expansion included a unique concept of enhanced biological phosphorus removal in combination with the traditional Triple Ditch process. Three years of operational data demonstrates the benefits of the enhanced recycle mode of operation and mixed liquor solids management.

The Triple-Ditch, often referred to as a T-Ditch, is a three-ditch system that operates without separate clarifiers. It is the modified BioDenitro process developed by Tholander in the early 1970’s (Tholander et al, 1976) as shown in Figure 1. The T-Ditch was initially developed to treat larger wastewater flow volumes that were not easily accomodated in vertical loop reactor ditch systems. The first and third oxidation ditches are designed to provide de-nitrification and settling by alternating the mode of operation. The middle ditch serves as a continuous aeration zone and provides flow distribution between the first and third ditch. The BioDenitro, T-Ditch process uses a two-phase, six-step operation and is schematically illustrated in Figure 1. The system is operated as a sequencing batch reaction process by internally moving the mixed liquor suspended solids between the outside ditches. Therefore, clarifiers are not required.

The T-Ditch system can be sized to provide a treatment capacity for the removal of carbon organics and nitrogen with nitrification and de-nitrification, but not enhanced biological phosphorus removal. To enhance the capability of biological phosphorus removal, the T-Ditch design was modified with a new concept of biological phosphorus removal (Yang, et al, 1999). Two Bio-P components were added to the front of the T-Ditch systems as shown in Figure 2. Mixed liquor recycle flows are constantly pumped from the bottom of each side-ditch near the openings, between the side-ditches and center ditch. The recycled flow is then directed to a sludge thickener. This process modification, shown in Figure 2, was part of the 1998 plant upgrade intended to achieve enhanced biological phosphorus removal.

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