diff --git a/src/watertap_contrib/reflo/unit_models/chemical_softening.py b/src/watertap_contrib/reflo/unit_models/chemical_softening.py
index ee60ed2a..c8891f92 100644
--- a/src/watertap_contrib/reflo/unit_models/chemical_softening.py
+++ b/src/watertap_contrib/reflo/unit_models/chemical_softening.py
@@ -175,7 +175,7 @@ def build(self):
         non_hardness_comps = [
             j
             for j in self.config.property_package.solute_set
-            if j not in ["Ca_2+", "Mg_2+"]
+            if j not in ["Ca_2+", "Mg_2+", "Alkalinity_2-"]
         ]
 
         # MW of components
@@ -1028,6 +1028,109 @@ def eq_mass_balance_mg(b):
                 to_units=pyunits.kg / pyunits.s,
             )
 
+        # TODO:
+        # single stage lime (example in book Crittenden): source water alkalinity - Ca hardness + residual Ca hardness
+        # excess lime (example in book Crittenden): source water alkalinity - Ca hardness - excess lime dose + residual Ca hardness
+        # single stage lime soda (only hydroxide alkalinity): source water alkalinity + soda ash - total hardness + residual hardness
+        # excess lime soda (only hydroxide alkalinity): source water alkalinity + soda ash - total hardness + residual hardness
+
+        if (
+            self.config.softening_procedure_type
+            is SofteningProcedureType.single_stage_lime
+        ):
+
+            @self.Constraint(doc="Alkalinity mass balance")
+            def eq_effluent_alk(b):
+                return b.properties_out[0].flow_mass_phase_comp[
+                    "Liq", "Alkalinity_2-"
+                ] == b.properties_in[0].flow_mass_phase_comp[
+                    "Liq", "Alkalinity_2-"
+                ] - pyunits.convert(
+                    b.Ca_CaCO3 * b.properties_in[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                ) + pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Ca_2+"]
+                    * b.Ca_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                )
+
+        elif self.config.softening_procedure_type is SofteningProcedureType.excess_lime:
+
+            @self.Constraint(doc="Alkalinity mass balance")
+            def eq_effluent_alk(b):
+                return b.properties_out[0].flow_mass_phase_comp[
+                    "Liq", "Alkalinity_2-"
+                ] == b.properties_in[0].flow_mass_phase_comp[
+                    "Liq", "Alkalinity_2-"
+                ] - pyunits.convert(
+                    b.total_hardness * b.properties_in[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                ) - pyunits.convert(
+                    b.excess_CaO * b.properties_in[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                ) + pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Ca_2+"]
+                    * b.Ca_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                ) + pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Ca_2+"]
+                    * b.Ca_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                ) + pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Mg_2+"]
+                    * b.Mg_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                )
+
+        elif (
+            self.config.softening_procedure_type
+            is SofteningProcedureType.single_stage_lime_soda
+        ):
+
+            @self.Constraint(doc="Alkalinity mass balance")
+            def eq_effluent_alk(b):
+                return b.properties_out[0].flow_mass_phase_comp[
+                    "Liq", "Alkalinity_2-"
+                ] == pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Ca_2+"]
+                    * b.Ca_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                )
+
+        elif (
+            self.config.softening_procedure_type
+            is SofteningProcedureType.excess_lime_soda
+        ):
+
+            @self.Constraint(doc="Alkalinity mass balance")
+            def eq_effluent_alk(b):
+                return b.properties_out[0].flow_mass_phase_comp[
+                    "Liq", "Alkalinity_2-"
+                ] == pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Ca_2+"]
+                    * b.Ca_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                ) + pyunits.convert(
+                    b.properties_out[0].conc_mass_phase_comp["Liq", "Mg_2+"]
+                    * b.Mg_CaCO3_conv
+                    * b.properties_out[0].flow_vol_phase["Liq"],
+                    to_units=pyunits.kg / pyunits.s,
+                )
+
+        @self.Constraint(doc="Alkalinity mass balance")
+        def eq_mass_balance_alk(b):
+            return (
+                b.properties_waste[0].flow_mass_phase_comp["Liq", "Alkalinity_2-"]
+                == b.properties_in[0].flow_mass_phase_comp["Liq", "Alkalinity_2-"]
+                - b.properties_out[0].flow_mass_phase_comp["Liq", "Alkalinity_2-"]
+            )
+
         if ["TSS"] in comps:
 
             @self.Constraint(doc="Sludge production")
diff --git a/src/watertap_contrib/reflo/unit_models/tests/test_chemical_softening.py b/src/watertap_contrib/reflo/unit_models/tests/test_chemical_softening.py
index 4b007399..104f1c1c 100644
--- a/src/watertap_contrib/reflo/unit_models/tests/test_chemical_softening.py
+++ b/src/watertap_contrib/reflo/unit_models/tests/test_chemical_softening.py
@@ -165,7 +165,7 @@ def test_build(self, chem_soft_frame):
             assert isinstance(port, Port)
             assert len(port.vars) == 3
 
-        assert number_variables(m) == 96
+        assert number_variables(m) == 95
         assert number_total_constraints(m) == 62
         assert number_unused_variables(m) == 17
 
@@ -206,7 +206,7 @@ def test_solution(self, chem_soft_frame):
         soft_results = {
             "ca_eff_target": 0.008,
             "mg_eff_target": 0.002427,
-            "removal_efficiency": {"SiO2": 0.7, "Alkalinity_2-": 0.7},
+            # "removal_efficiency": {"SiO2": 0.7, "Alkalinity_2-": 0.7},
             "volume_mixer": 1.0535825,
             "volume_floc": 65.8489,
             "volume_sed": 342.4143,
@@ -416,7 +416,7 @@ def test_build(self, chem_soft_frame):
             assert isinstance(port, Port)
             assert len(port.vars) == 3
 
-        assert number_variables(m) == 86
+        assert number_variables(m) == 85
         assert number_total_constraints(m) == 51
         assert number_unused_variables(m) == 18
 
@@ -458,7 +458,7 @@ def test_solution(self, chem_soft_frame):
         soft_results = {
             "ca_eff_target": 0.008,
             "mg_eff_target": 0.002427,
-            "removal_efficiency": {"Alkalinity_2-": 0.7},
+            # "removal_efficiency": {"Alkalinity_2-": 0.7},
             "volume_mixer": 13.89,
             "volume_floc": 868.29,
             "volume_sed": 4515.12,
@@ -672,7 +672,7 @@ def test_build(self, chem_soft_frame):
             assert isinstance(port, Port)
             assert len(port.vars) == 3
 
-        assert number_variables(m) == 86
+        assert number_variables(m) == 85
         assert number_total_constraints(m) == 52
         assert number_unused_variables(m) == 18
 
@@ -714,7 +714,7 @@ def test_solution(self, chem_soft_frame):
         soft_results = {
             "ca_eff_target": 0.012,
             "mg_eff_target": 0.000728,
-            "removal_efficiency": {"Alkalinity_2-": 0.7},
+            # "removal_efficiency": {"Alkalinity_2-": 0.7},
             "volume_mixer": 13.89,
             "volume_floc": 868.29,
             "volume_sed": 4515.12,
@@ -931,7 +931,7 @@ def test_build(self, chem_soft_frame):
             assert isinstance(port, Port)
             assert len(port.vars) == 3
 
-        assert number_variables(m) == 86
+        assert number_variables(m) == 85
         assert number_total_constraints(m) == 52
         assert number_unused_variables(m) == 18
 
@@ -973,7 +973,7 @@ def test_solution(self, chem_soft_frame):
         soft_results = {
             "ca_eff_target": 0.012,
             "mg_eff_target": 0.000728,
-            "removal_efficiency": {"Alkalinity_2-": 0.7},
+            # "removal_efficiency": {"Alkalinity_2-": 0.7},
             "volume_mixer": 13.89,
             "volume_floc": 868.29,
             "volume_sed": 4515.12,
@@ -1192,7 +1192,7 @@ def test_build(self, chem_soft_frame):
             assert isinstance(port, Port)
             assert len(port.vars) == 3
 
-        assert number_variables(m) == 96
+        assert number_variables(m) == 95
         assert number_total_constraints(m) == 62
         assert number_unused_variables(m) == 17