Try handling unclosed sql connections by calling dispose on the engine before it is put back into the pool.

cdplib/db_handlers/SQLHandler.py

@@ -34,9 +34,10 @@ class SQLHandlerPool(metaclass=SingletonThreadsafe):
             self._log.warning("Ran out of SQL handlers, 10 more have been added. Are you sure you've returned yours?")
         return self._sql_handlers.pop()
 
-    def release(self, mongodb_handler):
+    def release(self, sql_handler):
         if len(self._sql_handlers) < self._size:
-            self._sql_handlers.append(mongodb_handler)
+            sql_handler._engine.dispose()
+            self._sql_handlers.append(sql_handler)
 
 class SQLHandler:
     '''
@@ -501,7 +502,6 @@ class SQLHandler:
         :rtype: DataFrame
         '''
         try:
-            '''
             connection = self._engine.connect()
 
             data = pd.read_sql(sql=query,
@@ -509,10 +509,7 @@ class SQLHandler:
                                **read_sql_kwargs)
 
             connection.close()
-            '''
-            data = pd.read_sql(sql=query,
-                               con=self._engine,
-                               **read_sql_kwargs)
+           
             return data
 
         except Exception as e: