How to create the total reaction out of the two steps behind denitrification?
After balancing carbon and nitrogen, I'd recommend you go for balancing the charge in the equation:

$$ \ce{6NO3- + CH3OH -&gt; 3N2 + CO2 + H2O + 6OH-} $$

Now, we've to balance oxygen and hydrogen without disturbing the already balanced species. It would be interesting to note that here the stoichiometric coefficients of methanol and carbon dioxide should be the same or the number of carbon atoms wouldn't be equal on both sides of the equation.

So, let's assume the stoichiometric coefficient for both these two species to be $x$. Also, let's assume the stoichiometric coefficient of water on the right hand side to be $y$. The equation now looks like this:

$$ \ce{6NO3- + xCH3OH -&gt; 3N2 + xCO2 + yH2O + 6OH-} $$

We now balance the number of oxygen atoms:

$$ 18 + x = 2x + y + 6 $$

And the number of hydrogen atoms:

$$ 4x = 2y + 6 $$

Solving these two equations, you get $x=5$ and $y=7$. And therefore your overall equation finally is:

$$ \ce{6NO3- + 5CH3OH -&gt; 3N2 + 5CO2 + 7H2O + 6OH-} $$