By Malcolm Taylor, PhD, P.E.; WSSC Water; email@example.com and R.C. Brandt, PhD, P.E.; Material Matters, Inc.
The benefits of recycling biosolids as a soil amendment are well established. Product quality
is the major factor determining where and how biosolids are accepted for use, and by whom. Notably, experience has shown that product odor is the key characteristic presenting the greatest challenge to biosolids acceptance once regulatory standards are satisfied. Given this challenge, the Metropolitan Washington Council of Governments (MWCOG) launched a collaborative study involving DC Water; WSSC Water; Material Matters Inc.; and Penn State University; to investigate post-dewatered processing (PDP) enhancement strategies to mitigate biosolids product odor and improve handling characteristics.
Biosolids for the study were generated at DC Water’s Blue Plains Advanced Wastewater Treatment Plant. (AWWTP). This product (marketed as Bloom) is generated using a thermal hydrolysis/anaerobic digestion (THP/AD) process and produces Class A Exceptional Quality (EQ) biosolids in accordance with USEPA Part 503 standards. While fresh dewatered Bloom cake (~29% total solids) exhibits moderate to low Class A/EQ odor levels, MWCOG seeks to identify practical management strategies that further reduce odor to levels comparable with off-the-shelf, bagged soil amendment products that are routinely accepted by the public and sold locally through commercial retail/wholesale outlets.
The recently published WRF project report entitled High Quality Biosolids from Wastewater – NTRY7R15 (WRF, 2020) identified several strategies that help to mitigate biosolids product odors and assist in garnering public acceptance for beneficial use. Such practices include product de-gassing via agitation, aging, mixing with less odorous residual materials, and highlighting similarities with widely accepted commercial products. To this end, the strategy for this investigation involves evaluation of several windrow management practices incorporating alternative sheltering methods, windrow turning (agitation) frequency, mixing with aluminum-based water treatment residuals, and odor assessment relative to off-the-shelf bagged soil amendment products.
Site management, windrow turning, and sampling is managed by WSSC Water at one of their facilities near the Blue Plains AWWTP. The study focuses on the use of shallow windrows (~1.5-ft high x 8-ft wide) to maximize the exposed biosolids surface area. Uniform windrow agitation is accomplished using a Koomia Skid-TrnrTM unit mounted on a skid steer. Odor assessment (Penn State Odor Assessment Laboratory) and physical/chemical product characterization (Penn State Agricultural Analytical Services Laboratory) of samples are performed using standard methods. In addition, OxiTopTM and SOLVITATM compost maturity testing is conducted during each sampling event. Samples are collected from the various treatments throughout nine-week trials. To evaluate the influence of weather, three sets of seasonal mixing trials will be conducted. The Spring 2021 trial is ongoing, and the findings of the Summer 2020 are highlighted herein. Additional details of the study including a comprehensive overview of all results and subsequent PDP strategy recommendations will be summarized in a final report to be published at the conclusion of the study.
To create a consistent high-quality biosolids product marketed as consumer-ready, the handleability of the material must also be considered. Material that is either too sticky, too clumpy, too hard, or otherwise difficult to handle and clean up has minimal value as a marketable product. In addition to non- or minimally offensive odors, consumers desire a product that is consistent in nature, easy to work with, and easy to clean up. A biosolids handleability index was therefore developed to assist the research team in defining product quality with respect to handleability. The handleability index is an assimilation of several physical characteristics commonly used to define the properties of soil-like products. Characteristics include: stickiness, friability, smearing, staining, and dustiness (Figure 3). During each sampling event, material was scored against the index criteria to capture the change in material handleability over the nine-week trial period.
Summer 2020 trials evaluated three windrow shelter methods: Open-Air (no-shelter), Roof-Shelter (metal-skin pole barn), and Tarp-Covered piles. Windrow turning frequency treatments included: Zero-Turns, One-Turn/week, Two-Turns/week.
Summer 2020 Trial Findings (See Figure 1)
1. Odor concentration Detection Threshold (DT) levels for all treatments increased at the two-week age point but trended downward beyond that point going forward.
2. Odor concentration DT levels in windrow/turned Bloom cake were appreciably reduced in the nine-week aged material compared to the original Bloom cake delivered on Day-Zero (21 July 2020).
3. Odor concentration DT levels in windrow materials that were turned (agitated) were appreciably lower than DT levels found in windrows that were not agitated.
4. Labeled Magnitude Scale odor intensity (suprathreshold odor strength) and Hedonic Tone (pleasantness) reported for agitated windrows greatly improved over time. Notably, the Open-Air, two-turn windrow material exhibited a weak, musty-earth character, with an average reported hedonic tone of less than -0.1 (Hedonic Tone scale of +10 to -10). Odors of this nature are not typically considered offensive.
5. Regardless of turning frequency, Tarp-Covered windrow cake material retained more moisture than Open-Air and Under-Roof windrow treatments and retained the sticky paste-like consistency found in the original Bloom cake product delivered to the research site. As such, the handleability of the nine-week age Tarp-Cover window product would be considerably less attractive for consumers compared to the Open-Air and Under-Roof windrow product treatments. (see Figure 2)
6. Static pile (not agitated) Open-Air and Under-Roof windrows formed a hard surface crust that inhibited curing of underlying material. As such, these windrows produced a non-homogeneous product requiring significantly greater agitation to produce a consistent product with attractive handleability characteristics.
7. Product Respiratory Activity (RA), measured by OxitopTM trended downward over time for all treatments, with the lowest RA found in the Open-Air, two-turn windrow. This finding indicates the windrow-aged product became more stable (less biologically active) over time.
8. As listed in Figure 1, all non-biosolids bagged products showed DT levels less than 100. This DT appears to be a reasonable and readily achievable threshold using windrow aging. Notably, the two-turn Open-Air and Roof-Cover windrow treatments were successful in attaining these low odor DT levels with Bloom during Summer trials.
One of the most notable findings to-date is that despite numerous rainfall events occurring throughout the Summer trials (16.3 inches, over twice the annual average over this period) the turned, open-air piles had dried into a granular-like material after seven to nine weeks. After heavy rainfall (>~1”) the granular material did become wetter to the touch but did not revert to a cake-like material. One to two days after rain, material dried out to pre-rain levels and remained as a granular aggregate. This finding suggests that once chemically bonded waters of hydration are driven off, the material does not chemically rehydrate after a rainfall event, suggesting that processed material can be stored outside without risking progress made during curing. Although intriguing, this finding is preliminary and should be investigated further.
A unique element of this research involves the blending of biosolids with water treatment residuals (WTR) generated at WSSC Water’s Potomac Water Filtration Plant (Figure 4). Once mixed, the biosolids/WTR blend (1:1 / M/M) was managed in the same manner as biosolids windrows. Potential benefits of blending biosolids with WTR include: (1) Chemically bind excess soluble Phosphorus (P) in biosolids; (2) reduce biosolids odors; (3) Increase sand, silt, and clay fractions, and compactability; (4) improve moisture retention capacity. Preliminary findings from the Fall trial indicate that blending can be effective at reducing odors and improving product handleability. Details of this work will be presented in the final report.
Whereas not a focus of this study, another added benefit of PDP using agitation and aging is decreased product volume attributed to drying. Since biosolids hauling and application services are almost exclusively billed on a wet ton basis, reductions in moisture content may result in reduced disposal costs. Within seven weeks, material turned once a week and stored under roof dried from 32% total solids (TS) to a maximum 67% TS. In this instance the resulting wet tonnage of biosolids requiring hauling and application was essentially cut in half. The applicability of this type of management strategy is very clearly specific to each generator, nonetheless the potential for PDP to reduce overall management costs, while also improving product quality, is substantial.
Research findings to date indicate that post-dewatered processing using a combination of agitation and passive drying can substantially improve product quality and marketability of biosolids as a consumer-ready product. The ongoing spring trial will focus, in part, on reducing the time required to improve biosolids odor and handleability and better defining the conditions under which this can occur. The research team remains optimistic that methods developed in this study can aid utilities in developing a marketable product to reduce operating costs and promote beneficial reuse.
Acknowledgements: This project was made possible with funding from MWCOG and key contributions from multiple parties. Special thanks to: Karl Berger (MWCOG); James Fotouhi (DC Water); William Bayless and Ryan Smith (WSSC Water); Mike Hile (Penn State); Material Matters Inc.