## 1 Introduction  
Increasing populations mean there is an increased need for water suitable for human consumption and recreation while simultaneously negatively impacting the quality of water available. The increase in water demand, in conjunction with the growing frequency of extreme climate change events, has made the rapid prediction of population dynamics within an ecosystem essential for the effective management of harmful algal blooms (HABs) within drinking water reservoirs, recreational lakes and private lakes. HABs and the subsequent cyanotoxins they are able to produce pose a global threat to water bodies. For example, in 2008 over 35 states in the U.S. had documented HABs associated with cyanobacteria [1] with all 50 states reporting HAB events by 2015 [2]. [PERSON] et al. [3] reported HAB frequency in China increased by 40% each decade. HABs are a global issue impacting drinking water, recreation and aquaculture [4]). The increasing prevalence of potential HABs in lakes and reservoirs across the world has become a significant challenge for both in-lake management and toxin removal (e.g., in drinking water plants). Alum has been shown to bind phosphorus, both in water treatment and when used as an agent to bind legacy phosphorus in water bodies [5]. The proper operation of coagulation/flocculation, sedimentation and filtration steps in conventional surface water treatment plants has been found to be effective at removing cyanobacteria cells [6]. The most common coagulants used during the coagulation/sedimentation process to remove excessive phosphorus are aluminum sulfate (alum), and ferric chloride. The coagulants can only be used once and continuously produce a waste stream commonly referred to as a water treatment residual (WTR). Water treatment plants in the United States produce more than 2 million tons/year of WTR [7; 8]; with global WTR production reaching 10 million tons/day [9]. Processing and handling WTRs often require a significant portion of water treatment plant operating expenses [10]. Researchers have found that WTRs can be used to adsorb reactive phosphorous (PO4\({}^{3-}\)) which could lead to viable reuse of WTRs [11; 12; 13; 14; 15]. While source water protection and monitoring programs do assist with identification of sources and potential long term changes to reduce source water runoff, they do not address phosphorus (PO4\({}^{3-}\)) concentrations in incoming waterways. Even the best monitoring and protection programs, while able to capture in-stream PO4\({}^{3-}\) concentrations, often do not have a way to remove PO4\({}^{3-}\) or to quantify changes in in-stream PO4\({}^{3-}\) concentration reductions. One approach to HAB management that would address incoming PO4\({}^{3-}\) would be to remove excess nutrients levels in field as a pre-treatment step. WTR has the ability to capture PO4\({}^{3-}\) and has potential to be a cost effective method to capture and remove PO4\({}^{3-}\), in streams/tributaries prior to entering a reservoir, lake, or other HAB-prone body of water. Using WTRs upstream provides in-stream nutrient reduction, enables the quantification of PO4\({}^{3-}\) removal, and provides a beneficial reuse of a waste material. There have been several approaches investigated for reducing PO4\({}^{3-}\) upstream such direct addition of amendments to source water, wetlands or riparian buffers [16]. For instance, [PERSON] et al. [17] found that 5 mg/L of alum added directly to the inlet of Jameson Lake, Washington USA reduced phosphorus levels in the lake from 0.13 mg/L to below 0.02 mg/L. Although effective, this would require a constant addition of alum. Similarly, while constructed wetlands are effective at nutrient reduction, they are not widely implemented due to their large footprint, long residence time and clogging [18]. An approach to increase the effectiveness of constructed wetland's ability to remove phosphorus and offset some of the drawbacks is to use sorbents in constructed wetlands. [PERSON] et al.