Use of Wastewater Grit and Milorganite Chaff in Concrete

Wastewater grit is a major byproduct from preliminary municipal wastewater treatment. Chaff is a collection of dried biosolids generated as a waste product from Milorganite® fertilizer production. Because landfilling is a costly and unsustainable method, the development of technologies for diverting grit from landfills to a useful product by tapping grit’s inherent values (e.g., sand and gravel) are of great interest. Chaff, consisting of fibers, provides a potential source for bolstering concrete tensile strength through internal means. This research has proposed a concept of grit and chaff additive-based concrete, an expansion of the grit-assisted patch material developed (GAP) in previous Milwaukee Metropolitan Sewerage District (MMSD) sponsored projects. Calcium oxide, a main component in cement, with a high pH value, could efficiently inactivate pathogens in raw grit and keep it stable in the long term. Each additive group was found to be pathogen-free while maintaining acceptable concrete compressive strength. In this project, fresh and hardened concrete properties (e.g., slump, air content, compressive strength, and tensile strength) were tested after adding each additive to concrete mixes at differing percentages. It was found that the properties of fresh and hardened concrete were maintained with low loadings of both grit and chaff, while properties decreased with higher loadings. The consistency of each additive in terms of its properties (e.g., water content, absorption) was a cause for concern in affecting the properties of each mix, though this could be mitigated in a variety of ways. Grit and chaff affected the color of concrete in minute ways, shifting the general color slightly – though this change is only noticeable at further inspection and loses any noticeable difference after exposure to the environment. It was found that none of the additive groups harbored any amount of pathogenic substance, both by observational test and numerical test methods. This shows that there is no concern for potentially harmful materials being introduced through the implementation of grit or chaff in concrete, maintaining the safety of the public and those who work with the material. The specimens with regular and epoxy steel rebar were exposed to an outdoor environment for five months beginning in July and continuing until testing in December. The section of regular rebar outside of concrete corroded at similar level across all specimens, while the exposed portion of all epoxy coated bars remained corrosion free. There were no obvious signs of corrosion on the internal portion of rebar within all concrete types. Therefore, it can be concluded that grit and chaff in concrete do not cause additional corrosion in rebar compared to normal concrete given a similar exposure and time period. Although additional design and care are required to maintain similar characteristics of fresh and hardened concrete to those of normal concrete, grit and chaff-based concrete absorbs the social cost of wastewater byproduct contamination and has the potential to save municipalities large amounts of operating costs on previously necessary landfill costs while also contributing to the protection of the environment at large.