At a wastewater treatment plant, FeCl$3($s$)$ is added as a source of Fe$3+$ to bind excess phosphate to facilitate its removal by forming solid FePO$4($s$)$ as shown in the following reaction: FePO$4($s$)=$ Fe$3++$ PO$43-$ logKsp $=-26\mathrm{.}4$ What concentration of Fe$3+$ is needed to maintain the phosphate concentration below the regulatory limit of $1$ mg P$/$L$.$