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 and can be used for landscaping and flower beds. 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. 

Important Contacts
  • Compost Purchases
    • Email: sales@synagro.com, Phone: 800-370-0035
    • Matt Niccum: mniccum@synagro.com, Phone: 936-203-7191
  • Dillo Dirt General Questions
    • Email: dillodirt@austintexas.gov, Phone:512-972-1956
Dillo Dirt Uses and Application Rates 
Uses and application rates of Dillo Dirt compost for soil improvements
     Use  Compost
lbs per 1,000 sq ft ** (inches of depth)		 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.

To calculate cubic yards of Dillo Dirt™ needed:
area length (ft.) x width (ft.) x inches Dillo Dirt™ ÷ 324= cubic yards

* 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.

Biosolids Reuse Contractor Information 

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.

Guidance Documents for Composting Operations

Hornsby Bend BMP guidance documents are for public information and for use by the Contractor(s) who are working onsite at Hornsby Bend BMP with the composting operations.  These plans include Odor, Dust and Fire Control plans, a Spill Response Plan, and a Compost Sampling Plan.

The Treatment Process

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.

Anaerobic Digestion

  • 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.

Biosolids Composting and Treated Water Reuse

  • ​​​​​Water separated from the biosolids is treated in a 180-acre facultative pond system. The treated effluent is used to irrigate an onsite farm. Hay is harvested from this land by a contract farmer and the City receives a portion of the revenue.

    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.

Frequently Asked Questions

Do you have a Donation Program?

  • Several thousand tons of Dillo Dirt are available for donation to programs benefiting the citizens of Austin including public schools, parks, nonprofits, and other charitable organizations. The material is donated but the end user is responsible for transportation. Contact dillodirt@austintexas.gov for additional information.

Is Dillo Dirt safe to use for landscaping?

  • Yes. Dillo Dirt meets EPA and TCEQ requirements.

Why not landfill instead of recycling?

  • Treating and recycling biosolids and yard trimmings is very beneficial to the soil. It is more cost-effective to recycle than landfill these materials.

What about metals in the biosolids?

  • Biosolids contain small quantities of metals that travel through the sewer system. The EPA and the TCEQ have established acceptable levels of metals to protect public health and the environment. 

    Austin has a very aggressive and successful pretreatment program that monitors wastewater quality and controls industry discharges into the system. 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.

How do I use Dillo Dirt?

  • Please see the tab above with application rates and usage instructions.

How are my yard trimmings used for Dillo Dirt?

  • Yard trimmings are picked up during brush bulk pickup by ARR and ground into mulch at Hornsby Bend. Other sources of brush include the public drop off located at Hornsby Bend. After being ground, this brush is mixed with biosolids and composted.

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