diff --git a/biosteam/wastewater/high_rate/polishing_filter.py b/biosteam/wastewater/high_rate/polishing_filter.py
index 5fe45b20..e424f1a9 100644
--- a/biosteam/wastewater/high_rate/polishing_filter.py
+++ b/biosteam/wastewater/high_rate/polishing_filter.py
@@ -181,6 +181,14 @@ def _run(self):
 
         self.growth_rxns(mixed.mol)
         self.decomp_rxns.force_reaction(mixed.mol)
+        biogas.phase = air_in.phase = air_out.phase = 'g'
+        if self.filter_type == 'aerobic' and mixed.imol['O2'] < 0:
+            O2 = -mixed.imol['O2']
+            air_in.imol['O2'] = O2
+            air_in.imol['N2'] = 0.79/0.21 * O2
+            mixed.imol['O2'] = 0
+        else:
+            air_in.empty()
         mixed.split_to(eff, waste, self._isplit.data)
 
         sludge_conc = self._sludge_conc
@@ -191,15 +199,6 @@ def _run(self):
             waste.ivol['Water'] = m_insolubles/sludge_conc
             eff.ivol['Water'] += diff
 
-        biogas.phase = air_in.phase = air_out.phase = 'g'
-
-        if mixed.imol['O2'] < 0:
-            air_in.imol['O2'] = - mixed.imol['O2']
-            air_in.imol['N2'] = - 0.79/0.21 * mixed.imol['O2']
-            mixed.imol['O2'] = 0
-        else:
-            air_in.empty()
-
         if self.filter_type == 'anaerobic':
             degassing(eff, biogas)
             degassing(waste, biogas)
@@ -211,7 +210,6 @@ def _run(self):
             degassing(eff, air_out)
             degassing(waste, air_out)
             air_out.imol['N2'] += air_in.imol['N2']
-            air_out.imol['O2'] += air_in.imol['O2']
             self._recir_ratio = None
 
         if self.T is not None: biogas.T = eff.T = waste.T = air_out.T = self.T
@@ -637,4 +635,4 @@ def organic_rm(self):
         """[float] Overall organic (COD) removal rate."""
         Qi, Qe = self._inf.F_vol, self.outs[1].F_vol
         Si, Se = self.compute_COD(self._inf), self.compute_COD(self.outs[1])
-        return 1 - Qe*Se/(Qi*Si)
\ No newline at end of file
+        return 1 - Qe*Se/(Qi*Si)
diff --git a/tests/test_high_rate_wastewater.py b/tests/test_high_rate_wastewater.py
new file mode 100644
index 00000000..1421cdb0
--- /dev/null
+++ b/tests/test_high_rate_wastewater.py
@@ -0,0 +1,50 @@
+import biosteam as bst
+import pytest
+
+
+def test_aerobic_polishing_filter_satisfies_oxygen_deficit_before_split():
+    bst.main_flowsheet.clear()
+    chemicals = bst.Chemicals(['CO2', 'AceticAcid', 'H2O', 'O2', 'N2'])
+    bst.settings.set_thermo(
+        bst.wastewater.high_rate.append_wwt_chemicals(chemicals)
+    )
+    influent = bst.Stream(
+        'influent',
+        AceticAcid=1,
+        H2O=100,
+        units='kg/hr',
+    )
+    recycle = bst.Stream('recycle')
+    air = bst.Stream('air', phase='g')
+    unit = bst.wastewater.high_rate.PolishingFilter(
+        'PF',
+        ins=(influent, recycle, air),
+        outs=('biogas', 'effluent', 'waste', 'air_out'),
+        filter_type='aerobic',
+    )
+
+    unit.simulate()
+    assert unit.outs[1].imol['O2'] >= 0
+    assert unit.outs[2].imol['O2'] >= 0
+    
+    assert abs(unit.mass_balance_error()) < 1e-6
+
+
+def test_polishing_filter_rejects_invalid_filter_type():
+    bst.main_flowsheet.clear()
+    chemicals = bst.Chemicals(['CO2', 'AceticAcid', 'H2O', 'O2', 'N2'])
+    bst.settings.set_thermo(
+        bst.wastewater.high_rate.append_wwt_chemicals(chemicals)
+    )
+
+    with pytest.raises(ValueError, match="filter_type"):
+        bst.wastewater.high_rate.PolishingFilter(
+            'PF',
+            ins=('influent', 'recycle', 'air'),
+            outs=('biogas', 'effluent', 'waste', 'air_out'),
+            filter_type='aerobc',
+        )
+
+if __name__ == '__main__':
+    test_aerobic_polishing_filter_satisfies_oxygen_deficit_before_split()
+    test_polishing_filter_rejects_invalid_filter_type()
\ No newline at end of file