## 4 Results
### Impacts and Benefits of Ammonium Rinsing  
In the experiment, alkaline calcium-rich rinse solutions were injected in columns following bioccmentation to remove MICP byproducts. However, this post-treatment rinsing technique requires significant energy and materials, namely water and calcium chloride. This implies a tradeoff between the environmental benefits of NH\({}_{4}\)+ by-product removal and the life cycle impacts and costs of the rinsing technique. To investigate this tradeoff, the LCSA results were evaluated as a function of the byproduct removal that occurs with increasing rinsing effort (i.e., increasing injected rinse solution volume and rinsing injection time). Figure 5 shows the changes in indicator values as a result of rinsing relative to their values at the end of cementation (i.e., before rinsing). As shown, high masses of in situ nitrogen byproducts (near 450 g of N) are present immediately following bioccmentation, with reductions in these values occurring in all columns with increases in applied rinse injections. With increasing rinsing effort, all environmental impacts and cost indicators followed one of the following three observed trends: (1) global warming potential, smog formation potential, and social cost decreased slightly and reached a minimum value after injecting between 0.9 and 1.8 PV (45 to 90 L) of rinse solution but subsequently increased as rinsing became less effective at removing byproducts at low aqueous NH\({}_{4}\)+ concentrations; (2) eutrophication potential progressively decreased and reached a minimum value after injecting nearly 5.4 PV (270 L) of rinse solutions; or (3) all other indicators increased minimally at first and then increased more substantially as rinsing became less effective after injecting about 5.4 PV (270 L) of rinse solution. As with any tradeoff, an optimal solution is one that best satisfies the objective functions. Although reductions in nitrogen byproducts from initial values \(\approx\)450 g N to \(\approx\)20 g N in all columns resulted in near linear reductions in eutrophication potential, global warming potential illustrated a more complex behavior, achieving a minimum as nitrogen byproducts decreased to between \(\approx\)50 g N and \(\approx\)100 g N but with large increases below \(\approx\)20 g N. This behavior illustrated the tension between the benefits afforded by rinsing, which can effectively remove large nitrogen byproduct masses at the start of rinsing, and the drawbacks of rinsing observed at larger injection volumes (lower nitrogen byproduct concentrations), wherein a more limited removal of byproducts occur. At these low nitrogen concentrations, large increases in costs and impacts are observed, despite likely being practically necessary to meet site water quality requirements. Considering all impact categories, these trends collectively suggest that injecting between 1.8 and 3.6 PV (90 L to 180 L) of rinse solution, thereby reducing nitrogen byproducts to between 4% (18 g N) and 38% (171 g N) of their initial values in each column, can minimize the life cycle impacts and costs associated with post-treatment rinsing. In all further comparisons considered in this study, the injection of 1.8 PV (90 L) of rinse solution will be considered the optimal scenario, which minimizes process impacts. However, as mentioned previously, site regulations may necessitate further removal to achieve nitrogen byproduct concentrations below what can be obtained with this optimal strategy. At these larger injection volumes, results from the meter-scale experiments demonstrate that injecting 10.5 PV (525 L) of rinse solution can reduce total in situ N by over 97.5% in all columns, resulting in average aqueous NH4\({}^{+}\) concentrations of 3.5 mM, 1.5 mM, and 3.8 mM NH4\({}^{+}\) for Columns 1 through 3, respectively, albeit at the expense of substantially larger process impacts. These post-rinsing concentrations are slightly higher but on the same order of magnitude as those which may be required at many sites throughout the U.S. [22].