Compost is a soil product manufactured through the controlled aerobic, biological decomposition of biodegradable materials.
Dillo Dirt™ is compost made at Hornsby Bend Biosolids Management Plant ( Hornsby Bend). It was the first program of its kind in the state and one of the oldest in the nation. Brush generated by City of Austin departments and residents is mixed with treated biosolids (wastewater sludge) and composted to create Dillo Dirt. The heat generated in composting (130 to 170 degrees Fahrenheit) is sufficient to virtually eliminate pathogens. After active composting for over a month, Dillo Dirt is "cured" for several months and screened to produce finished compost. Dillo Dirt is made from totally recycled materials which diverts waste from the landfill and reduces costs to citizens.
Dillo Dirt™ meets all Texas Commission on Environmental Quality ( TCEQ ) and Environmental Protection Agency (EPA) requirements for unrestricted use including vegetable gardens. Like other composts, Dillo Dirt has many benefits such as reducing watering requirements and enriching the soil environment.
You can drop off brush at the Hornsby Bend facility to be used to make Dillo Dirt. Click here for hours and instructions for drop-off.
| Use |
Compost |
Instructions |
|---|---|---|
| Turfgrasses: Establishment Soil Incorporated |
1500 to 4000 lbs (1/2" - 2") | Mix with top 4"-6" of soil. Use lower rate on fertile soil, higher rate on infertile soil. |
| Surface Mulch | 700 to 4000 lbs (1/4" -2") |
Spread evenly on surface before seeding small species (Bermuda) or after seeding large seeded species (fescues. |
| Maintenance | 300 to 700 lbs (1/10"-1/4") |
Spread evenly on surface. On cool season grasses, either apply high rate in fall or lower rate in fall and spring. |
| Nursery Crop & Ornamentals (shrubs & trees) Establishment |
1500 to 4300 lbs (1/2"- 2 1/4") |
Mix with top 6"-8" of soil. Do not use where acidic-soil plants (azalea, rhodendron, etc) are to be grown). |
| Maintenance | 300 to 600 lbs (1/10" - 1/5") |
Spread evenly on surface soil. Can be mixed into soil or used as mulch. |
| Tree Balling | Use at 50% rate | Mix based on compost with bark or soil. |
| Potting Soil Mixes | Not more than 1/3 by volume | Water and drain mixes several times before planting to prevent salt injury to plants. Blending materials: peat, sand, perlite, vermiculite, and bark. |
* Source: International Process Systems, Inc. Glastonbury, Connecticut
**1,500 lbs. per 1,000 square feet is approximately equal to 1/2 inch depth of compost.
Compost has a bulk density of about 1,000 lbs. per cubic yard and 40% moisture.
Source: based on USDA publication ARM-NE-6, August 1979.
The City of Austin has a long-term contract with Synagro of Texas-CDR, Inc. (Synagro) for the composting of biosolids produced at Hornsby Bend. This contract was competitively bid and awarded on November 1, 2018. Synagro utilizes the Hornsby Bend facility for composting operations and produces Dillo Dirt™ under Austin Water’s oversight. In addition to Dillo Dirt, Synagro also produces AllGro® compost. AllGro meets all regulatory specifications for biosolids compost but is not required to meet the same quality specifications as Dillo Dirt.
Originally established in the 1950’s as a series of stabilization ponds used to treat biosolids from the City's wastewater plants, the Hornsby Bend Biosolids Management Plant has become an award-winning, nationally recognized biosolids recycling facility.
Situated on 1,200 acres of land along the lower Colorado River, about eight miles east of downtown Austin, the facility is a model for innovative approaches to benefit the environment, such as reducing waste, producing compost, and protecting ecosystems.
Biosolids (specifically primary and secondary waste-activated sludge) are pumped to Hornsby Bend from their sources at Walnut Creek and South Austin Regional wastewater treatment plants. The biosolids are then thickened by centrifuges to reduce volume and allow more time for treatment in the digesters.
Once thickened, biosolids are stabilized by a process called anaerobic (without oxygen) digestion. The process occurs in eight two-million-gallon tanks called digesters. Digestion destroys more than 90% of the pathogens present in the biosolids. Methane gas is a by-product of the process and is consumed by onsite generators to produce electricity. After digestion, biosolids are thickened again by belt presses before they are composted.
The dewatered biosolids are combined with chipped brush and composted in windrows measuring more than 500 feet long. Yard trimmings are picked up by Austin Resource Recovery (ARR) and ground into mulch at Hornsby Bend. After being ground, this material is mixed with biosolids for composting. The biological activity and heat from composting produces a stable organic product that can be safely used by the public. This compost is marketed and sold by the biosolids reuse contractor.
Dillo Dirt contains far less metals than the allowable maximums for unrestricted use. Some of these metals, including zinc, molybdenum, and chromium, are essential nutrients for plants.