Treatment of Industrial Sludge Using A Mobile Pilot Sludge Treatment Reed Bed System

Dow Inc. (Dow) St. Charles Operations (SCO) is a 2,000-acre integrated petro-chemical manufacturing complex located near New Orleans, Louisiana. Biosolids generated at the SCO wastewater treatment facility (WWTF) are dewatered using two belt filter presses to 12% to 14% dry solids (DS). The filter cake is disposed of in an off-site landfill located 115 km from SCO. Dow is exploring alternate methods of dewatering WWTF biosolids due to an increase in disposal costs and limited landfill capacity. One process that shows promise as an environment-friendly and a cost-effective alternative is a sludge treatment wetland also known as a Sludge Treatment Reed Bed (STRB) system. Dow initially screened two potential sludge sources using a capillary suction test (CST) and determined each was a viable candidate for pilot testing. Based on promising CST results and the site being located in a subtropical climate suitable for STRB, Dow authorized Plummer Associates and WSP Denmark to design and construct a STRB pilot system built on a flat-bed trailer allowing testing at multiple facilities in the US. The pilot system would be used to test the suitability of a STRB as a process and identify the size requirements and cost of a full-scale system. The pilot system was commissioned in February 2020 and consists of five STRB basins (1.1 m2), two sludge holding tanks with sludge transfer pumps, and a filtrate drain and monitoring system. Each basin was vegetated with common reed (Phragmites australis). A 3-month Phase 1 test showed promising results for the two sludge sources that were tested. Air stabilized sludge (0.6 – 0.8 % DS) and clarifier sludge (1.1 to 1.4% DS) were intermittently loaded at 80 to 120 liters/load to the basins and efficiently drained within 2-4 hrs from starting the load. Peak dewatering rates ranged from 0.006 to 0.040 liters/sec/m2 and 0.002 to 0.035 liters/sec/m2 for air stabilized and clarifier sludge, respectively. The filtrate quality was excellent, meeting the discharge permit parameters for the facility, and initial hydraulic drainage characteristics were re-established after resting periods varying from 14 to 60 days.
Phase 2 was initiated in July 2020 pursuing more intensive test regimes and higher loading rates to determine the full system design criteria. Several challenges were experienced during Phase 2, some of which are unique to the SCO site. For example, the pilot system has weathered four hurricanes, including Ida with winds at 150 miles per hour. Biotic pressures included infestation of red imported fire ants (Solenopsis invicta) and non-native Roseau Cane Scale (Nipponaclerda biwakoensis). Fire ant colonies did not appear to affect the drainage characteristics of the basins. The scale, however, caused significant die-back of P. australis such that three of the five basins had to be replanted with Giant bulrush (Schoenoplectus californicus), which is resistant to the scale. Basins with the bulrush have functioned comparable to those with P. australis. The industrial biosolids were found to be low in nutrients, resulting in the need to fertilize with nitrogen. Air stabilization sludge continued to drain efficiently with a volume reduction of 93% – 96% and DS content up to 71% to 82% after 15 days resting. Intermittent use of polymer in the clarifiers and from the belt filter press significantly inhibited the drainage efficiency of the sludge within the STRB. Phase 2 testing will be completed in October 2021.
Based on the initial results, the STRB is a valid technology option. The STRB system, a nature-based solution, also aligns with Dow's 2025 Sustainability Goals such as the Nature Goal. It presents a reduced overall energy footprint; returns filtrate water to the local watershed by avoiding transport and disposal of the water in the landfill; decreases the hydraulic load to the WWTF; reduces overall materials associated with biosolids treatment and handling; and provides either the opportunity for beneficial reuse of the mineralized biosolids or reduced volume to landfill.