edit /var/lib/zentyal/conf/apache2.conf
and set
<IfModule mpm_prefork_module>
    StartServers             1
    MinSpareServers          10
    MaxSpareServers          20
    MaxClients               1
    MaxRequestsPerChild  10000
</IfModule>

# worker MPM

<IfModule mpm_worker_module>
    StartServers             1
    MinSpareThreads          10
    MaxSpareThreads          20
    ThreadsPerChild          1
    MaxClients               1
    MaxRequestsPerChild  10000
</IfModule> 
then save
:wq
and like i said you should create a hook for the webserver module (postsetconf) to keep this at every restart.thats all folks.Regards