DC Water Bloom: A biosolids for land application success story

After being the first utility in the US to adopt advanced anaerobic digestion with Cambi's thermal hydrolysis process (THP) technology, DC Water knew that it had a great biosolids product on hand.

The utility then launched a long-term grassroots campaign that changed the public perception of its biosolids in the US capital and other key towns. Today, the Class A biosolids from its Blue Plains facility represent a growing business for the company, sold under the Bloom brand. What can be learned from Bloom's success, and what strategies did the organization employ to make its biosolids product attractive for so many applications?

The switch to Class A biosolids with thermal hydrolysis

Prior to 2000, DC Water's Blue Plains wastewater treatment plant used anaerobic digestion to produce Class B biosolids. At the turn of the century, the utility began researching new biosolids strategies, prompted in part by a growing serviced base and ageing digesters that would shortly be shut down. While thorough studies on anaerobic digestion and dewatering options were being conducted, the plant used high-solids centrifuges to dewater its sludge, followed by lime stabilization. The resulting Class B biosolids were given away for use in agricultural and forest land, timber and pulp forests grown on mine recultivation sites, and reclamation projects.

During this period, the plant faced challenges regarding its biosolids' odor and final outlets. There were limited spaces for land application in Washington DC, which prompted the utility to haul the material out to farms in its neighbouring jurisdictions, Virginia and Maryland. The transportation of the limed material contributed significantly to a cost of approximately $ 19 million per year, representing around 21% of the plant's operational expenses. The biosolids' odor level and the cost of the new sludge strategy were, therefore, crucial factors in the selection of the new sludge technology for Blue Plains.

Cambi THP system at DC Water

The THP system at DC Water's Blue Plains facility

DC Water concluded in 2008 that advanced digestion, specifically Cambi's thermal hydrolysis process (THP) with mesophilic anaerobic digestion (MAD), provided the best scenario for sludge treatment because it would achieve a low odor, high-quality biosolids cake in lower quantities. This would allow the utility to diversify its handling options while significantly cutting the upfront capital expenditure for anaerobic digesters and monthly operational expenses for the sludge.

Up-close biosolids land application for the community

During the extensive upgrade of the plant, which lasted several years, DC Water launched a public relations campaign to grow the variety of outlets for the expected high-quality biosolids cake, including public contact sites like parks, gardens, and residential areas.

The plant composted its biosolids off-site to resemble the expected high-quality product from the advanced digestion upgrade. These composted biosolids were used in the following (all images are courtesy of DC Water and Bloom):

  • A small community garden using the composted biosolids product was made on-site at Blue Plains for visitors and guests to see applications of the final product.
  • DC Water also established strong relationships with community gardeners in all eight wards of Washington, DC (representing about 70 community gardens). Biosolids were given to these gardens for free, including school and youth gardens. The utility would also give out flowers or produce from such plots during public meetings.
  • DC Water presented the product in relevant spaces and events, heightening the brand's visibility. The composted biosolids were also used in city restoration projects managed by the Department of Transportation.

From biosolids bane to boon: The rise of DC Water Bloom

The new advanced anaerobic digestion system started operations in 2015. One year later, DC Water used its company subsidiary Blue Drop to commercially launch the system's resulting Class A (Exceptional Quality) biosolids product under the trade name "Bloom". Since 2016, the organization has been expanding its product portfolio by making various blends, with plans to develop the product range over a decade. The Bloom product now represents significant savings in biosolids handling expenses and an additional revenue source for DC Water.

Bloom currently has four varieties (see picture below). Fresh Bloom (top left) is 100% made up of Blue Plains' dewatered biosolids, mostly bought for use on farmland, wetlands, and public works. Cured Bloom (top right) is the same biosolids product but air-dried to 60% dry solids before it's sold. It is made available in 50-pound bags sold in retail stores but can also be bought in bulk from the plant. Lastly, Bloom blends – a sand sawdust blend and a wood blend (bottom left and right) are sold in bulk, typically for public contact sites or landscape architecture.

 DC Water Bloom soil products from biosolids

Bloom products. Clockwise from top left: Fresh Bloom, Cured Bloom, Bloom wood blend, Bloom sand sawdust blend. Source: DC Water

The Bloom brand's success in its first few years was made possible by an extensive campaign, including introducing the brand to key public officials, the release of some of the products in retail outlets, a website launch, and continued studies on the applications of the product. DC Water asked schools and community gardens for feedback on the material.

Apart from allowing them to create a remarkable soil product, the switch to advanced anaerobic digestion using THP, along with investments in other technologies, helped the plant achieve outstanding results:

  • increased renewable energy covering one-third of the need on-site
  • a 40% reduction in overall carbon footprint
  • lowered biosolids volumes to almost 30% of the original output

 The new technologies ultimately save Blue Plains $ 14 million per year.

Bloom biosolids studies: Testaments to an effective and sustainable soil product

Through various studies, Bloom showed remarkable effects on plant growth and soil health in multiple ecosystems. A few of these studies are summarised below:

1. Bloom biosolids improve roadside-like soil, supporting turfgrass growth

Roadside soil typically suffers from compaction, erosion, and poor vegetation, and turfgrass is a common plant used for its recovery. A study by the University of Maryland (2016-2018) set out to determine the effects of Bloom products and alternatives to turfgrass and loamy soil.

  land plot before biosolids applicationland plot after biosolids application

Plots simulating roadside soil during the application of biosolids and fertilizer products (left) and after a growing season (right) in a study conducted by the University of Maryland. Source: Bloom

This study found that Bloom products increased soil permeability and compaction after two growing seasons, contributing to better root growth, water and solute movement, and soil aeration. , making them suitable for promoting turfgrass growth.

2. Bloom biosolids store large amounts of carbon in a short amount of time and are beneficial in the restoration of degraded urban soils

A study conducted at the Virginia Polytechnic Institute and State University aimed to determine the effects of BLOOM biosolids and other treatments on degraded urban soil and vegetable yield. The biosolids amendments produced better soil chemistry and showed a capacity to sequester large amounts of soil carbon during the two years of the study.

Despite the study's harsh soil conditions, the tested biosolids treatments produced vegetable yields comparable with inorganic fertilizer and better yields when applied at reclamation rates (five times the agronomic rate).

3. Bloom biosolids outperform other organic amendments in wetland restoration

A study by Brian Scott for the University of Maryland compared the methane emissions of various organic amendments for wetland restoration and found that Bloom produced the least amount of methane and at a comparable rate with natural soils that received no amendment. It also stimulated plant growth in the wetland without encouraging the growth of weeds.

Cured bloom was therefore recommended as a better amendment for wetland restoration than other amendments such as manure, hay, or wood chips.


DC Water's use of thermal hydrolysis for advanced anaerobic digestion and its accompanying public relations and research strategy have created immense public confidence in its biosolids. As a result, the utility is also now open to licensing the Bloom brand.

Bloom has demonstrated that thermal hydrolysis is not only a cost-effective option for wastewater solids management in the long run but can also help change a community's mindset. With the right strategy, utilities can communicate the safety and efficacy of biosolids as fertilizer or soil products in our daily lives.

Though thermal hydrolysis makes biosolids suited for a range of outlets like incineration, sludge drying, and other thermal processes, land application is still the most common endpoint for hydrolyzed sludge. Want to find out which other plants use it to create soil amendments or biosolids fertilizer? Check out our references page.

19 April 2024


Cambi - Multiple contributors

This article is the effort of various authors within Cambi.

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