It seems like the capacity ...

It seems like the capacity of your anaerobic system is not enough to deal with the high organic load from the pig farm. I suggest an aerobic tank or lined pond with fine and ultrafine bubble generation to facilitate the composition of organic matter. 

The most effective way would be to implement a membrane bio-reactor, this will provide BOD >5mg/L and COD >30mg/L and remove N and P as well. This essentially condenses an anaerobic and aerobic tank into one system and removes the need for a settling pond or clarifier by using membrane separation to achieve the desired filtration.

Please email me at andrew@waterandoilsolutions.com.au, I am experienced in agriculture and food processing wastewater treatment in Australia. I am Vietnamese background, so happy to speak in Vietnamese if necessary. Next time I am in Vietnam visiting family and friends, maybe we can catch up and introduce each other. I am eager to see if I can help the Vietnamese water industry in any way.